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Benefit Plans | 12 Months Ended
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Dec. 31, 2017
Benefit Plans | BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows:
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.66% |  | 4.19% |  | 4.47% |  | 3.53% |  | 3.98% |  | 4.30%
Discount rate for net periodic cost | 4.19% |  | 4.47% |  | 4.18% |  | 3.98% |  | 4.30% |  | 3.97%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 5.80% |  | 6.30% |  | 6.20%
Rate of compensation increase | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.68% |  | 4.22% |  | 4.50% |  | 3.51% |  | 3.95% |  | 4.28%
Discount rate for net periodic cost | 4.22% |  | 4.50% |  | 4.20% |  | 3.95% |  | 4.28% |  | 3.94%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 6.20% |  | 6.60% |  | 6.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.69% |  | 4.23% |  | 4.51% |  | 3.51% |  | 3.96% |  | 4.28%
Discount rate for net periodic cost | 4.23% |  | 4.51% |  | 4.20% |  | 3.96% |  | 4.28% |  | 3.96%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 3.50% |  | 4.70% |  | 4.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements.
Alliant Energy | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $12.5 |  |  |  | $12.6 |  |  |  | $15.9 |  |  |  | $5.0 |  |  |  | $5.3 |  |  |  | $5.5 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 53.6 |  |  | 8.6 |  |  | 9.4 |  |  | 9.1 |
Expected return on plan assets (a) | (65.5 | ) |  | (65.5 | ) |  | (75.0 | ) |  | (6.1 | ) |  | (6.1 | ) |  | (8.4 | )
Amortization of prior service credit (b) | (0.4 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (4.1 | ) |  | (11.3 | )
Amortization of actuarial loss (c) | 37.6 |  |  | 37.4 |  |  | 35.4 |  |  | 3.8 |  |  | 4.7 |  |  | 4.8 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
Settlement losses (d) | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $36.1 |  |  |  | $37.2 |  |  |  | $30.2 |  |  |  | $11.1 |  |  |  | $9.2 |  |  |  | ($0.3 | )
IPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $7.3 |  |  |  | $7.5 |  |  |  | $8.8 |  |  |  | $2.1 |  |  |  | $2.3 |  |  |  | $2.4 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 25.0 |  |  | 3.5 |  |  | 3.8 |  |  | 3.8 |
Expected return on plan assets (a) | (30.8 | ) |  | (30.9 | ) |  | (35.8 | ) |  | (4.3 | ) |  | (4.3 | ) |  | (5.7 | )
Amortization of prior service credit (b) | (0.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | — |  |  | (2.6 | ) |  | (6.1 | )
Amortization of actuarial loss (c) | 16.1 |  |  | 16.5 |  |  | 15.3 |  |  | 2.0 |  |  | 2.6 |  |  | 2.3 |
|  | $15.9 |  |  |  | $17.4 |  |  |  | $13.2 |  |  |  | $3.3 |  |  |  | $1.8 |  |  |  | ($3.3 | )
WPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $4.9 |  |  |  | $4.9 |  |  |  | $5.8 |  |  |  | $1.9 |  |  |  | $2.0 |  |  |  | $2.1 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 22.6 |  |  | 3.4 |  |  | 3.8 |  |  | 3.7 |
Expected return on plan assets (a) | (28.5 | ) |  | (28.3 | ) |  | (32.4 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (1.5 | )
Amortization of prior service cost (credit) (b) | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (0.2 | ) |  | (0.9 | ) |  | (3.5 | )
Amortization of actuarial loss (c) | 18.5 |  |  | 17.6 |  |  | 16.8 |  |  | 1.6 |  |  | 1.8 |  |  | 2.2 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
|  | $16.8 |  |  |  | $16.7 |  |  |  | $13.5 |  |  |  | $5.9 |  |  |  | $5.9 |  |  |  | $3.0 |
(a) | The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
(b) | Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.
(c) | Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
(d) | Settlement losses related to payments made to retired executives of Alliant Energy.
The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Defined Benefit |  |  |  | Defined Benefit |  |  |  | Defined Benefit |  |
| Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans
Actuarial loss |  | $35.2 |  |  |  | $3.4 |  |  |  | $15.0 |  |  |  | $1.2 |  |  |  | $17.2 |  |  |  | $2.0 |
Prior service credit | (0.7 | ) |  | (0.2 | ) |  | (0.2 | ) |  | — |  |  | (0.2 | ) |  | (0.2 | )
|  | $34.5 |  |  |  | $3.2 |  |  |  | $14.8 |  |  |  | $1.2 |  |  |  | $17.0 |  |  |  | $1.8 |
Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $1,244.3 |  |  |  | $1,206.3 |  |  |  | $220.1 |  |  |  | $221.4 |
Service cost | 12.5 |  |  | 12.6 |  |  | 5.0 |  |  | 5.3 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 8.6 |  |  | 9.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Actuarial (gain) loss | 83.6 |  |  | 48.3 |  |  | 5.4 |  |  | (0.3 | )
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Net benefit obligation at December 31 | 1,303.1 |  |  | 1,244.3 |  |  | 222.3 |  |  | 220.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 895.7 |  |  | 895.0 |  |  | 105.8 |  |  | 106.9 |
Actual return on plan assets | 136.7 |  |  | 74.3 |  |  | 12.9 |  |  | 8.2 |
Employer contributions | 6.6 |  |  | 2.3 |  |  | 9.2 |  |  | 6.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Fair value of plan assets at December 31 | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Under funded status at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $8.8 |  |  |  | $3.2 |
Current liabilities | (2.2 | ) |  | (6.5 | ) |  | (9.1 | ) |  | (8.6 | )
Pension and other benefit obligations | (350.2 | ) |  | (342.1 | ) |  | (110.9 | ) |  | (108.9 | )
Net amounts recognized at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: |  |  |  |  |  |  |
Net actuarial loss |  | $509.1 |  |  |  | $535.1 |  |  |  | $47.4 |  |  |  | $52.6 |
Prior service credit | (6.5 | ) |  | (6.9 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $502.6 |  |  |  | $528.2 |  |  |  | $46.1 |  |  |  | $51.1 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $570.4 |  |  |  | $556.1 |  |  |  | $90.1 |  |  |  | $91.3 |
Service cost | 7.3 |  |  | 7.5 |  |  | 2.1 |  |  | 2.3 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 3.5 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Actuarial (gain) loss | 34.9 |  |  | 19.1 |  |  | (0.1 | ) |  | (0.7 | )
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Net benefit obligation at December 31 | 592.9 |  |  | 570.4 |  |  | 89.4 |  |  | 90.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 422.0 |  |  | 422.7 |  |  | 68.2 |  |  | 69.2 |
Actual return on plan assets | 64.3 |  |  | 35.3 |  |  | 8.9 |  |  | 5.3 |
Employer contributions | 0.6 |  |  | 0.8 |  |  | 2.0 |  |  | 0.3 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Fair value of plan assets at December 31 | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Under funded status at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $5.9 |  |  |  | $0.4 |
Current liabilities | (0.5 | ) |  | (0.7 | ) |  | (2.0 | ) |  | (1.9 | )
Pension and other benefit obligations | (148.7 | ) |  | (147.7 | ) |  | (20.4 | ) |  | (20.4 | )
Net amounts recognized at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $218.9 |  |  |  | $233.6 |  |  |  | $18.7 |  |  |  | $25.4 |
Prior service credit | (2.1 | ) |  | (2.3 | ) |  | — |  |  | — |
|  | $216.8 |  |  |  | $231.3 |  |  |  | $18.7 |  |  |  | $25.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $529.2 |  |  |  | $505.9 |  |  |  | $88.9 |  |  |  | $89.7 |
Service cost | 4.9 |  |  | 4.9 |  |  | 1.9 |  |  | 2.0 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 3.4 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Actuarial loss | 38.3 |  |  | 25.7 |  |  | 4.1 |  |  | 0.5 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Net benefit obligation at December 31 | 559.8 |  |  | 529.2 |  |  | 90.4 |  |  | 88.9 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 389.7 |  |  | 386.8 |  |  | 18.6 |  |  | 18.7 |
Actual return on plan assets | 59.6 |  |  | 32.4 |  |  | 1.2 |  |  | 1.2 |
Employer contributions | 0.1 |  |  | 0.1 |  |  | 6.8 |  |  | 5.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Fair value of plan assets at December 31 | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Under funded status at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $2.9 |  |  |  | $2.7 |
Current liabilities | (0.1 | ) |  | (0.1 | ) |  | (6.8 | ) |  | (6.4 | )
Pension and other benefit obligations | (144.7 | ) |  | (139.4 | ) |  | (67.8 | ) |  | (66.6 | )
Net amounts recognized at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $224.7 |  |  |  | $236.1 |  |  |  | $23.6 |  |  |  | $21.5 |
Prior service credit | (1.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $223.2 |  |  |  | $234.7 |  |  |  | $22.3 |  |  |  | $20.0 |
Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $1,269.0 |  |  |  | $1,201.5 |  |  |  | $222.3 |  |  |  | $220.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 1,269.0 |  |  | 1,201.5 |  |  | 222.3 |  |  | 220.1 |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 1,303.1 |  |  | 1,244.3 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $573.1 |  |  |  | $546.7 |  |  |  | $89.4 |  |  |  | $90.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 573.1 |  |  | 546.7 |  |  | 89.4 |  |  | 90.1 |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 592.9 |  |  | 570.4 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $548.1 |  |  |  | $513.2 |  |  |  | $90.4 |  |  |  | $88.9 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 548.1 |  |  | 513.2 |  |  | 90.4 |  |  | 88.9 |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 559.8 |  |  | 529.2 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | N/A |  |  | N/A |
In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions):
| IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016
Regulatory assets |  | $38.9 |  |  |  | $37.3 |  |  |  | $28.1 |  |  |  | $30.0 |
Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $6.3 |  |  |  | $4.4 |  |  |  | $0.3 |
OPEB plans | 9.0 |  |  | 2.0 |  |  | 6.8 |
(a) | Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions):
Alliant Energy | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $72.2 |  |  |  | $73.9 |  |  |  | $76.4 |  |  |  | $77.1 |  |  |  | $92.9 |  |  |  | $398.2 |
OPEB | 18.2 |  |  | 18.4 |  |  | 17.7 |  |  | 17.5 |  |  | 17.2 |  |  | 80.1 |
|  | $90.4 |  |  |  | $92.3 |  |  |  | $94.1 |  |  |  | $94.6 |  |  |  | $110.1 |  |  |  | $478.3 |
IPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $33.9 |  |  |  | $33.8 |  |  |  | $36.5 |  |  |  | $36.6 |  |  |  | $37.6 |  |  |  | $188.4 |
OPEB | 7.1 |  |  | 7.1 |  |  | 7.2 |  |  | 7.1 |  |  | 7.0 |  |  | 32.6 |
|  | $41.0 |  |  |  | $40.9 |  |  |  | $43.7 |  |  |  | $43.7 |  |  |  | $44.6 |  |  |  | $221.0 |
WPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $31.4 |  |  |  | $32.0 |  |  |  | $32.4 |  |  |  | $32.4 |  |  |  | $32.7 |  |  |  | $167.8 |
OPEB | 8.3 |  |  | 8.3 |  |  | 7.5 |  |  | 7.3 |  |  | 7.0 |  |  | 31.8 |
|  | $39.7 |  |  |  | $40.3 |  |  |  | $39.9 |  |  |  | $39.7 |  |  |  | $39.7 |  |  |  | $199.6 |
Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017, the current target ranges and actual allocations for the defined benefit pension plan assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 3%
Equity securities - U.S. | 11 | % | - | 41% |  | 24%
Equity securities - international | 14 | % | - | 34% |  | 23%
Global asset securities | 5 | % | - | 15% |  | 10%
Risk parity securities | 5 | % | - | 15% |  | 10%
Fixed income securities | 20 | % | - | 40% |  | 30%
Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 1%
Equity securities - U.S. | 0 | % | - | 50% |  | 26%
Equity securities - international | 0 | % | - | 34% |  | 10%
Fixed income securities | 20 | % | - | 100% |  | 63%
Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $28.2 |  |  |  | $4.5 |  |  |  | $23.7 |  |  |  | $— |  |  |  | $30.4 |  |  |  | $5.0 |  |  |  | $25.4 |  |  |  | $— |
Equity securities - U.S. | 158.3 |  |  | 158.3 |  |  | — |  |  | — |  |  | 183.6 |  |  | 183.6 |  |  | — |  |  | — |
Equity securities - international | 137.5 |  |  | 137.5 |  |  | — |  |  | — |  |  | 97.4 |  |  | 97.4 |  |  | — |  |  | — |
Global asset securities | 49.4 |  |  | 49.4 |  |  | — |  |  | — |  |  | 53.0 |  |  | 53.0 |  |  | — |  |  | — |
Fixed income securities | 135.9 |  |  | 55.8 |  |  | 80.1 |  |  | — |  |  | 125.4 |  |  | 53.6 |  |  | 71.8 |  |  | — |
Total assets in fair value hierarchy | 509.3 |  |  |  | $405.5 |  |  |  | $103.8 |  |  |  | $— |  |  | 489.8 |  |  |  | $392.6 |  |  |  | $97.2 |  |  |  | $— |
Assets measured at net asset value | 441.1 |  |  |  |  |  |  |  |  | 405.9 |  |  |  |  |  |  |
Accrued investment income | 1.0 |  |  |  |  |  |  |  |  | 1.1 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.7 | ) |  |  |  |  |  |  |  | (1.1 | ) |  |  |  |  |  |
Total pension plan assets |  | $950.7 |  |  |  |  |  |  |  |  |  | $895.7 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $13.2 |  |  |  | $2.2 |  |  |  | $11.0 |  |  |  | $— |  |  |  | $14.4 |  |  |  | $2.4 |  |  |  | $12.0 |  |  |  | $— |
Equity securities - U.S. | 73.9 |  |  | 73.9 |  |  | — |  |  | — |  |  | 86.5 |  |  | 86.5 |  |  | — |  |  | — |
Equity securities - international | 64.2 |  |  | 64.2 |  |  | — |  |  | — |  |  | 45.9 |  |  | 45.9 |  |  | — |  |  | — |
Global asset securities | 23.0 |  |  | 23.0 |  |  | — |  |  | — |  |  | 24.9 |  |  | 24.9 |  |  | — |  |  | — |
Fixed income securities | 63.4 |  |  | 26.0 |  |  | 37.4 |  |  | — |  |  | 59.1 |  |  | 25.3 |  |  | 33.8 |  |  | — |
Total assets in fair value hierarchy | 237.7 |  |  |  | $189.3 |  |  |  | $48.4 |  |  |  | $— |  |  | 230.8 |  |  |  | $185.0 |  |  |  | $45.8 |  |  |  | $— |
Assets measured at net asset value | 205.8 |  |  |  |  |  |  |  |  | 191.2 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $443.7 |  |  |  |  |  |  |  |  |  | $422.0 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $12.3 |  |  |  | $2.0 |  |  |  | $10.3 |  |  |  | $— |  |  |  | $13.3 |  |  |  | $2.2 |  |  |  | $11.1 |  |  |  | $— |
Equity securities - U.S. | 69.1 |  |  | 69.1 |  |  | — |  |  | — |  |  | 79.9 |  |  | 79.9 |  |  | — |  |  | — |
Equity securities - international | 60.0 |  |  | 60.0 |  |  | — |  |  | — |  |  | 42.4 |  |  | 42.4 |  |  | — |  |  | — |
Global asset securities | 21.6 |  |  | 21.6 |  |  | — |  |  | — |  |  | 23.0 |  |  | 23.0 |  |  | — |  |  | — |
Fixed income securities | 59.3 |  |  | 24.3 |  |  | 35.0 |  |  | — |  |  | 54.5 |  |  | 23.3 |  |  | 31.2 |  |  | — |
Total assets in fair value hierarchy | 222.3 |  |  |  | $177.0 |  |  |  | $45.3 |  |  |  | $— |  |  | 213.1 |  |  |  | $170.8 |  |  |  | $42.3 |  |  |  | $— |
Assets measured at net asset value | 192.5 |  |  |  |  |  |  |  |  | 176.6 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $415.0 |  |  |  |  |  |  |  |  |  | $389.7 |  |  |  |  |  |  |
At December 31, the fair values of OPEB plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $1.2 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $— |  |  |  | $3.5 |  |  |  | $2.0 |  |  |  | $1.5 |  |  |  | $— |
Equity securities - U.S. | 27.9 |  |  | 27.9 |  |  | — |  |  | — |  |  | 22.5 |  |  | 22.5 |  |  | — |  |  | — |
Equity securities - international | 11.4 |  |  | 11.4 |  |  | — |  |  | — |  |  | 13.5 |  |  | 13.5 |  |  | — |  |  | — |
Global asset securities | 0.4 |  |  | 0.4 |  |  | — |  |  | — |  |  | 16.5 |  |  | 16.5 |  |  | — |  |  | — |
Fixed income securities | 66.6 |  |  | 66.0 |  |  | 0.6 |  |  | — |  |  | 46.8 |  |  | 46.2 |  |  | 0.6 |  |  | — |
Total assets in fair value hierarchy | 107.5 |  |  |  | $106.4 |  |  |  | $1.1 |  |  |  | $— |  |  | 102.8 |  |  |  | $100.7 |  |  |  | $2.1 |  |  |  | $— |
Assets measured at net asset value | 3.6 |  |  |  |  |  |  |  |  | 3.0 |  |  |  |  |  |  |
Total OPEB plan assets |  | $111.1 |  |  |  |  |  |  |  |  |  | $105.8 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $— |  |  |  | $0.8 |  |  |  | $0.8 |  |  |  | $— |  |  |  | $— |
Equity securities - U.S. | 22.3 |  |  | 22.3 |  |  | — |  |  | — |  |  | 17.0 |  |  | 17.0 |  |  | — |  |  | — |
Equity securities - international | 7.5 |  |  | 7.5 |  |  | — |  |  | — |  |  | 11.0 |  |  | 11.0 |  |  | — |  |  | — |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 7.0 |  |  | 7.0 |  |  | — |  |  | — |
Fixed income securities | 42.8 |  |  | 42.8 |  |  | — |  |  | — |  |  | 32.4 |  |  | 32.4 |  |  | — |  |  | — |
Total OPEB plan assets |  | $72.9 |  |  |  | $72.9 |  |  |  | $— |  |  |  | $— |  |  |  | $68.2 |  |  |  | $68.2 |  |  |  | $— |  |  |  | $— |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.6 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $1.3 |  |  |  | $— |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 5.5 |  |  | 5.5 |  |  | — |  |  | — |
Fixed income securities | 18.1 |  |  | 18.1 |  |  | — |  |  | — |  |  | 11.1 |  |  | 11.1 |  |  | — |  |  | — |
Total OPEB plan assets |  | $18.7 |  |  |  | $18.4 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $18.6 |  |  |  | $17.3 |  |  |  | $1.3 |  |  |  | $— |
For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016. 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016, respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  |  | 2016 |  |  | 2015 |  |  | 2017 |  | 2016 |  | 2015
401(k) costs |  | $24.8 |  |  |  | $23.6 |  |  |  | $24.9 |  |  |  | $12.8 |  |  |  | $12.0 |  |  |  | $12.7 |  |  |  | $11.1 |  |  |  | $10.7 |  |  |  | $11.2 |
(b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017, performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017, performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Compensation expense |  | $15.1 |  |  |  | $18.0 |  |  |  | $10.7 |  |  |  | $8.3 |  |  |  | $9.5 |  |  |  | $5.7 |  |  |  | $6.4 |  |  |  | $7.9 |  |  |  | $4.7 |
Income tax benefits | 6.2 |  |  | 7.4 |  |  | 4.4 |  |  | 3.4 |  |  | 4.0 |  |  | 2.4 |  |  | 2.6 |  |  | 3.2 |  |  | 1.9 |
As of December 31, 2017, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million, $3.1 million and $2.2 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three-year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Nonvested awards, January 1 | 257,599 |  |  | 288,430 |  |  | 288,848 |  |  | 93,320 |  |  | 116,412 |  |  | 127,330 |
Granted | 65,350 |  |  | 68,585 |  |  | 90,806 |  |  | 21,558 |  |  | 23,918 |  |  | 35,674 |
Vested | (99,438 | ) |  | (98,186 | ) |  | (91,224 | ) |  | (37,395 | ) |  | (42,760 | ) |  | (45,690 | )
Forfeited | — |  |  | (1,230 | ) |  | — |  |  | (5,746 | ) |  | (4,250 | ) |  | (902 | )
Nonvested awards, December 31 | 223,511 |  |  | 257,599 |  |  | 288,430 |  |  | 71,737 |  |  | 93,320 |  |  | 116,412 |
Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
| 2014 Grant |  | 2013 Grant |  | 2012 Grant |  | 2014 Grant |  | 2013 Grant |  | 2012 Grant
Performance awards vested | 99,438 |  | 98,186 |  | 91,224 |  | 37,395 |  | 42,760 |  | 45,690
Percentage of target number of performance awards | 147.5% |  | 165.0% |  | 167.5% |  | 147.5% |  | 165.0% |  | 167.5%
Aggregate payout value (in millions) | $5.6 |  | $5.1 |  | $5.1 |  | $1.5 |  | $1.7 |  | $1.6
Payout - cash (in millions) | $5.1 |  | $2.9 |  | $3.2 |  | $1.5 |  | $1.7 |  | $1.6
Payout - common stock shares issued | 5,185 |  | 22,408 |  | 21,950 |  | N/A |  | N/A |  | N/A
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017, by year of grant, were as follows:
| Performance Shares |  | Performance Units
| 2017 Grant |  | 2016 Grant |  | 2015 Grant |  | 2017 Grant |  | 2016 Grant |  | 2015 Grant
Nonvested awards at target | 65,350 |  |  | 67,355 |  |  | 90,806 |  |  | 18,600 |  |  | 21,227 |  |  | 31,910 |
Alliant Energy common stock closing price on December 29, 2017 |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  | N/A
Alliant Energy common stock closing price on grant date | N/A |  | N/A |  | N/A |  | N/A |  | N/A |  |  | $32.55 |
Estimated payout percentage based on performance criteria | 105 | % |  | 150 | % |  | 138 | % |  | 105 | % |  | 150 | % |  | 138 | %
Fair values of each nonvested award |  | $44.74 |  |  |  | $63.92 |  |  |  | $58.80 |  |  |  | $44.74 |  |  |  | $63.92 |  |  |  | $44.92 |
Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three-year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017, the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows:
| 2017 |  | 2016
| Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 67,355 |  |  |  | $33.96 |  |  | — |  |  |  | $— |
Granted | 65,350 |  |  | 39.12 |  |  | 68,585 |  |  | 33.96 |
Forfeited | — |  |  | — |  |  | (1,230 | ) |  | 33.90 |
Nonvested units, December 31 | 132,705 |  |  | 36.50 |  |  | 67,355 |  |  | 33.96 |
Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three-year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017, the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows:
| 2017 |  | 2016
Nonvested units, January 1 | 57,736 |  |  | — |
Granted | 56,013 |  |  | 58,790 |
Forfeited | — |  |  | (1,054 | )
Nonvested units, December 31 | 113,749 |  |  | 57,736 |
Performance-Contingent Restricted Stock and Performance-Contingent Cash Awards - As of December 31, 2017, the amount of nonvested performance-contingent restricted stock and performance-contingent cash awards was not material. (c) Deferred Compensation Plan - Alliant Energy maintains a DCP under which key employees may defer up to 100% of base salary and short-term cash incentive compensation and directors may elect to defer all or part of their retainer and committee fees. Key employees who have made the maximum allowed contribution to the Alliant Energy 401(k) Savings Plan may receive an additional credit to the DCP. Key employees and directors may elect to have their deferrals credited to a company stock account, an interest account, equity accounts or mutual fund accounts based on certain benchmark funds. Company Stock Account - The DCP does not permit diversification of deferrals credited to the company stock account and all distributions from participants’ company stock accounts are made in the form of shares of Alliant Energy common stock. The deferred compensation obligations for participants’ company stock accounts are recorded in “Additional paid-in capital” and the shares of Alliant Energy common stock held in a rabbi trust to satisfy this obligation are recorded in “Shares in deferred compensation trust” on Alliant Energy’s balance sheets. At December 31, the carrying value of the deferred compensation obligation for the company stock account and the shares in the deferred compensation trust based on the historical value of the shares of Alliant Energy common stock contributed to the rabbi trust, and the fair market value of the shares held in the rabbi trust, were as follows (in millions):
| 2017 |  | 2016
Carrying value |  | $11.1 |  |  |  | $10.0 |
Fair market value | 19.7 |  |  | 16.7 |
Interest, Equity and Mutual Fund Accounts - Distributions from participants’ interest, equity and mutual fund accounts are in the form of cash payments. The deferred compensation obligations for participants’ interest, equity and mutual fund accounts are recorded in “Pension and other benefit obligations” on the balance sheets. At December 31, 2017 and 2016, the carrying value of Alliant Energy’s deferred compensation obligations for participants’ interest, equity and mutual fund accounts, which approximates fair market value, was $21.8 million and $19.4 million, respectively.
IPL [Member] |
Benefit Plans | BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows:
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.66% |  | 4.19% |  | 4.47% |  | 3.53% |  | 3.98% |  | 4.30%
Discount rate for net periodic cost | 4.19% |  | 4.47% |  | 4.18% |  | 3.98% |  | 4.30% |  | 3.97%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 5.80% |  | 6.30% |  | 6.20%
Rate of compensation increase | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.68% |  | 4.22% |  | 4.50% |  | 3.51% |  | 3.95% |  | 4.28%
Discount rate for net periodic cost | 4.22% |  | 4.50% |  | 4.20% |  | 3.95% |  | 4.28% |  | 3.94%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 6.20% |  | 6.60% |  | 6.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.69% |  | 4.23% |  | 4.51% |  | 3.51% |  | 3.96% |  | 4.28%
Discount rate for net periodic cost | 4.23% |  | 4.51% |  | 4.20% |  | 3.96% |  | 4.28% |  | 3.96%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 3.50% |  | 4.70% |  | 4.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements.
Alliant Energy | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $12.5 |  |  |  | $12.6 |  |  |  | $15.9 |  |  |  | $5.0 |  |  |  | $5.3 |  |  |  | $5.5 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 53.6 |  |  | 8.6 |  |  | 9.4 |  |  | 9.1 |
Expected return on plan assets (a) | (65.5 | ) |  | (65.5 | ) |  | (75.0 | ) |  | (6.1 | ) |  | (6.1 | ) |  | (8.4 | )
Amortization of prior service credit (b) | (0.4 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (4.1 | ) |  | (11.3 | )
Amortization of actuarial loss (c) | 37.6 |  |  | 37.4 |  |  | 35.4 |  |  | 3.8 |  |  | 4.7 |  |  | 4.8 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
Settlement losses (d) | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $36.1 |  |  |  | $37.2 |  |  |  | $30.2 |  |  |  | $11.1 |  |  |  | $9.2 |  |  |  | ($0.3 | )
IPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $7.3 |  |  |  | $7.5 |  |  |  | $8.8 |  |  |  | $2.1 |  |  |  | $2.3 |  |  |  | $2.4 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 25.0 |  |  | 3.5 |  |  | 3.8 |  |  | 3.8 |
Expected return on plan assets (a) | (30.8 | ) |  | (30.9 | ) |  | (35.8 | ) |  | (4.3 | ) |  | (4.3 | ) |  | (5.7 | )
Amortization of prior service credit (b) | (0.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | — |  |  | (2.6 | ) |  | (6.1 | )
Amortization of actuarial loss (c) | 16.1 |  |  | 16.5 |  |  | 15.3 |  |  | 2.0 |  |  | 2.6 |  |  | 2.3 |
|  | $15.9 |  |  |  | $17.4 |  |  |  | $13.2 |  |  |  | $3.3 |  |  |  | $1.8 |  |  |  | ($3.3 | )
WPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $4.9 |  |  |  | $4.9 |  |  |  | $5.8 |  |  |  | $1.9 |  |  |  | $2.0 |  |  |  | $2.1 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 22.6 |  |  | 3.4 |  |  | 3.8 |  |  | 3.7 |
Expected return on plan assets (a) | (28.5 | ) |  | (28.3 | ) |  | (32.4 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (1.5 | )
Amortization of prior service cost (credit) (b) | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (0.2 | ) |  | (0.9 | ) |  | (3.5 | )
Amortization of actuarial loss (c) | 18.5 |  |  | 17.6 |  |  | 16.8 |  |  | 1.6 |  |  | 1.8 |  |  | 2.2 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
|  | $16.8 |  |  |  | $16.7 |  |  |  | $13.5 |  |  |  | $5.9 |  |  |  | $5.9 |  |  |  | $3.0 |
(a) | The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
(b) | Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.
(c) | Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
(d) | Settlement losses related to payments made to retired executives of Alliant Energy.
The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Defined Benefit |  |  |  | Defined Benefit |  |  |  | Defined Benefit |  |
| Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans
Actuarial loss |  | $35.2 |  |  |  | $3.4 |  |  |  | $15.0 |  |  |  | $1.2 |  |  |  | $17.2 |  |  |  | $2.0 |
Prior service credit | (0.7 | ) |  | (0.2 | ) |  | (0.2 | ) |  | — |  |  | (0.2 | ) |  | (0.2 | )
|  | $34.5 |  |  |  | $3.2 |  |  |  | $14.8 |  |  |  | $1.2 |  |  |  | $17.0 |  |  |  | $1.8 |
Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $1,244.3 |  |  |  | $1,206.3 |  |  |  | $220.1 |  |  |  | $221.4 |
Service cost | 12.5 |  |  | 12.6 |  |  | 5.0 |  |  | 5.3 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 8.6 |  |  | 9.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Actuarial (gain) loss | 83.6 |  |  | 48.3 |  |  | 5.4 |  |  | (0.3 | )
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Net benefit obligation at December 31 | 1,303.1 |  |  | 1,244.3 |  |  | 222.3 |  |  | 220.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 895.7 |  |  | 895.0 |  |  | 105.8 |  |  | 106.9 |
Actual return on plan assets | 136.7 |  |  | 74.3 |  |  | 12.9 |  |  | 8.2 |
Employer contributions | 6.6 |  |  | 2.3 |  |  | 9.2 |  |  | 6.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Fair value of plan assets at December 31 | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Under funded status at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $8.8 |  |  |  | $3.2 |
Current liabilities | (2.2 | ) |  | (6.5 | ) |  | (9.1 | ) |  | (8.6 | )
Pension and other benefit obligations | (350.2 | ) |  | (342.1 | ) |  | (110.9 | ) |  | (108.9 | )
Net amounts recognized at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: |  |  |  |  |  |  |
Net actuarial loss |  | $509.1 |  |  |  | $535.1 |  |  |  | $47.4 |  |  |  | $52.6 |
Prior service credit | (6.5 | ) |  | (6.9 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $502.6 |  |  |  | $528.2 |  |  |  | $46.1 |  |  |  | $51.1 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $570.4 |  |  |  | $556.1 |  |  |  | $90.1 |  |  |  | $91.3 |
Service cost | 7.3 |  |  | 7.5 |  |  | 2.1 |  |  | 2.3 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 3.5 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Actuarial (gain) loss | 34.9 |  |  | 19.1 |  |  | (0.1 | ) |  | (0.7 | )
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Net benefit obligation at December 31 | 592.9 |  |  | 570.4 |  |  | 89.4 |  |  | 90.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 422.0 |  |  | 422.7 |  |  | 68.2 |  |  | 69.2 |
Actual return on plan assets | 64.3 |  |  | 35.3 |  |  | 8.9 |  |  | 5.3 |
Employer contributions | 0.6 |  |  | 0.8 |  |  | 2.0 |  |  | 0.3 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Fair value of plan assets at December 31 | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Under funded status at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $5.9 |  |  |  | $0.4 |
Current liabilities | (0.5 | ) |  | (0.7 | ) |  | (2.0 | ) |  | (1.9 | )
Pension and other benefit obligations | (148.7 | ) |  | (147.7 | ) |  | (20.4 | ) |  | (20.4 | )
Net amounts recognized at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $218.9 |  |  |  | $233.6 |  |  |  | $18.7 |  |  |  | $25.4 |
Prior service credit | (2.1 | ) |  | (2.3 | ) |  | — |  |  | — |
|  | $216.8 |  |  |  | $231.3 |  |  |  | $18.7 |  |  |  | $25.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $529.2 |  |  |  | $505.9 |  |  |  | $88.9 |  |  |  | $89.7 |
Service cost | 4.9 |  |  | 4.9 |  |  | 1.9 |  |  | 2.0 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 3.4 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Actuarial loss | 38.3 |  |  | 25.7 |  |  | 4.1 |  |  | 0.5 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Net benefit obligation at December 31 | 559.8 |  |  | 529.2 |  |  | 90.4 |  |  | 88.9 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 389.7 |  |  | 386.8 |  |  | 18.6 |  |  | 18.7 |
Actual return on plan assets | 59.6 |  |  | 32.4 |  |  | 1.2 |  |  | 1.2 |
Employer contributions | 0.1 |  |  | 0.1 |  |  | 6.8 |  |  | 5.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Fair value of plan assets at December 31 | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Under funded status at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $2.9 |  |  |  | $2.7 |
Current liabilities | (0.1 | ) |  | (0.1 | ) |  | (6.8 | ) |  | (6.4 | )
Pension and other benefit obligations | (144.7 | ) |  | (139.4 | ) |  | (67.8 | ) |  | (66.6 | )
Net amounts recognized at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $224.7 |  |  |  | $236.1 |  |  |  | $23.6 |  |  |  | $21.5 |
Prior service credit | (1.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $223.2 |  |  |  | $234.7 |  |  |  | $22.3 |  |  |  | $20.0 |
Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $1,269.0 |  |  |  | $1,201.5 |  |  |  | $222.3 |  |  |  | $220.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 1,269.0 |  |  | 1,201.5 |  |  | 222.3 |  |  | 220.1 |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 1,303.1 |  |  | 1,244.3 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $573.1 |  |  |  | $546.7 |  |  |  | $89.4 |  |  |  | $90.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 573.1 |  |  | 546.7 |  |  | 89.4 |  |  | 90.1 |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 592.9 |  |  | 570.4 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $548.1 |  |  |  | $513.2 |  |  |  | $90.4 |  |  |  | $88.9 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 548.1 |  |  | 513.2 |  |  | 90.4 |  |  | 88.9 |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 559.8 |  |  | 529.2 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | N/A |  |  | N/A |
In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions):
| IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016
Regulatory assets |  | $38.9 |  |  |  | $37.3 |  |  |  | $28.1 |  |  |  | $30.0 |
Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $6.3 |  |  |  | $4.4 |  |  |  | $0.3 |
OPEB plans | 9.0 |  |  | 2.0 |  |  | 6.8 |
(a) | Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions):
Alliant Energy | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $72.2 |  |  |  | $73.9 |  |  |  | $76.4 |  |  |  | $77.1 |  |  |  | $92.9 |  |  |  | $398.2 |
OPEB | 18.2 |  |  | 18.4 |  |  | 17.7 |  |  | 17.5 |  |  | 17.2 |  |  | 80.1 |
|  | $90.4 |  |  |  | $92.3 |  |  |  | $94.1 |  |  |  | $94.6 |  |  |  | $110.1 |  |  |  | $478.3 |
IPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $33.9 |  |  |  | $33.8 |  |  |  | $36.5 |  |  |  | $36.6 |  |  |  | $37.6 |  |  |  | $188.4 |
OPEB | 7.1 |  |  | 7.1 |  |  | 7.2 |  |  | 7.1 |  |  | 7.0 |  |  | 32.6 |
|  | $41.0 |  |  |  | $40.9 |  |  |  | $43.7 |  |  |  | $43.7 |  |  |  | $44.6 |  |  |  | $221.0 |
WPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $31.4 |  |  |  | $32.0 |  |  |  | $32.4 |  |  |  | $32.4 |  |  |  | $32.7 |  |  |  | $167.8 |
OPEB | 8.3 |  |  | 8.3 |  |  | 7.5 |  |  | 7.3 |  |  | 7.0 |  |  | 31.8 |
|  | $39.7 |  |  |  | $40.3 |  |  |  | $39.9 |  |  |  | $39.7 |  |  |  | $39.7 |  |  |  | $199.6 |
Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017, the current target ranges and actual allocations for the defined benefit pension plan assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 3%
Equity securities - U.S. | 11 | % | - | 41% |  | 24%
Equity securities - international | 14 | % | - | 34% |  | 23%
Global asset securities | 5 | % | - | 15% |  | 10%
Risk parity securities | 5 | % | - | 15% |  | 10%
Fixed income securities | 20 | % | - | 40% |  | 30%
Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 1%
Equity securities - U.S. | 0 | % | - | 50% |  | 26%
Equity securities - international | 0 | % | - | 34% |  | 10%
Fixed income securities | 20 | % | - | 100% |  | 63%
Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $28.2 |  |  |  | $4.5 |  |  |  | $23.7 |  |  |  | $— |  |  |  | $30.4 |  |  |  | $5.0 |  |  |  | $25.4 |  |  |  | $— |
Equity securities - U.S. | 158.3 |  |  | 158.3 |  |  | — |  |  | — |  |  | 183.6 |  |  | 183.6 |  |  | — |  |  | — |
Equity securities - international | 137.5 |  |  | 137.5 |  |  | — |  |  | — |  |  | 97.4 |  |  | 97.4 |  |  | — |  |  | — |
Global asset securities | 49.4 |  |  | 49.4 |  |  | — |  |  | — |  |  | 53.0 |  |  | 53.0 |  |  | — |  |  | — |
Fixed income securities | 135.9 |  |  | 55.8 |  |  | 80.1 |  |  | — |  |  | 125.4 |  |  | 53.6 |  |  | 71.8 |  |  | — |
Total assets in fair value hierarchy | 509.3 |  |  |  | $405.5 |  |  |  | $103.8 |  |  |  | $— |  |  | 489.8 |  |  |  | $392.6 |  |  |  | $97.2 |  |  |  | $— |
Assets measured at net asset value | 441.1 |  |  |  |  |  |  |  |  | 405.9 |  |  |  |  |  |  |
Accrued investment income | 1.0 |  |  |  |  |  |  |  |  | 1.1 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.7 | ) |  |  |  |  |  |  |  | (1.1 | ) |  |  |  |  |  |
Total pension plan assets |  | $950.7 |  |  |  |  |  |  |  |  |  | $895.7 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $13.2 |  |  |  | $2.2 |  |  |  | $11.0 |  |  |  | $— |  |  |  | $14.4 |  |  |  | $2.4 |  |  |  | $12.0 |  |  |  | $— |
Equity securities - U.S. | 73.9 |  |  | 73.9 |  |  | — |  |  | — |  |  | 86.5 |  |  | 86.5 |  |  | — |  |  | — |
Equity securities - international | 64.2 |  |  | 64.2 |  |  | — |  |  | — |  |  | 45.9 |  |  | 45.9 |  |  | — |  |  | — |
Global asset securities | 23.0 |  |  | 23.0 |  |  | — |  |  | — |  |  | 24.9 |  |  | 24.9 |  |  | — |  |  | — |
Fixed income securities | 63.4 |  |  | 26.0 |  |  | 37.4 |  |  | — |  |  | 59.1 |  |  | 25.3 |  |  | 33.8 |  |  | — |
Total assets in fair value hierarchy | 237.7 |  |  |  | $189.3 |  |  |  | $48.4 |  |  |  | $— |  |  | 230.8 |  |  |  | $185.0 |  |  |  | $45.8 |  |  |  | $— |
Assets measured at net asset value | 205.8 |  |  |  |  |  |  |  |  | 191.2 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $443.7 |  |  |  |  |  |  |  |  |  | $422.0 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $12.3 |  |  |  | $2.0 |  |  |  | $10.3 |  |  |  | $— |  |  |  | $13.3 |  |  |  | $2.2 |  |  |  | $11.1 |  |  |  | $— |
Equity securities - U.S. | 69.1 |  |  | 69.1 |  |  | — |  |  | — |  |  | 79.9 |  |  | 79.9 |  |  | — |  |  | — |
Equity securities - international | 60.0 |  |  | 60.0 |  |  | — |  |  | — |  |  | 42.4 |  |  | 42.4 |  |  | — |  |  | — |
Global asset securities | 21.6 |  |  | 21.6 |  |  | — |  |  | — |  |  | 23.0 |  |  | 23.0 |  |  | — |  |  | — |
Fixed income securities | 59.3 |  |  | 24.3 |  |  | 35.0 |  |  | — |  |  | 54.5 |  |  | 23.3 |  |  | 31.2 |  |  | — |
Total assets in fair value hierarchy | 222.3 |  |  |  | $177.0 |  |  |  | $45.3 |  |  |  | $— |  |  | 213.1 |  |  |  | $170.8 |  |  |  | $42.3 |  |  |  | $— |
Assets measured at net asset value | 192.5 |  |  |  |  |  |  |  |  | 176.6 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $415.0 |  |  |  |  |  |  |  |  |  | $389.7 |  |  |  |  |  |  |
At December 31, the fair values of OPEB plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $1.2 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $— |  |  |  | $3.5 |  |  |  | $2.0 |  |  |  | $1.5 |  |  |  | $— |
Equity securities - U.S. | 27.9 |  |  | 27.9 |  |  | — |  |  | — |  |  | 22.5 |  |  | 22.5 |  |  | — |  |  | — |
Equity securities - international | 11.4 |  |  | 11.4 |  |  | — |  |  | — |  |  | 13.5 |  |  | 13.5 |  |  | — |  |  | — |
Global asset securities | 0.4 |  |  | 0.4 |  |  | — |  |  | — |  |  | 16.5 |  |  | 16.5 |  |  | — |  |  | — |
Fixed income securities | 66.6 |  |  | 66.0 |  |  | 0.6 |  |  | — |  |  | 46.8 |  |  | 46.2 |  |  | 0.6 |  |  | — |
Total assets in fair value hierarchy | 107.5 |  |  |  | $106.4 |  |  |  | $1.1 |  |  |  | $— |  |  | 102.8 |  |  |  | $100.7 |  |  |  | $2.1 |  |  |  | $— |
Assets measured at net asset value | 3.6 |  |  |  |  |  |  |  |  | 3.0 |  |  |  |  |  |  |
Total OPEB plan assets |  | $111.1 |  |  |  |  |  |  |  |  |  | $105.8 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $— |  |  |  | $0.8 |  |  |  | $0.8 |  |  |  | $— |  |  |  | $— |
Equity securities - U.S. | 22.3 |  |  | 22.3 |  |  | — |  |  | — |  |  | 17.0 |  |  | 17.0 |  |  | — |  |  | — |
Equity securities - international | 7.5 |  |  | 7.5 |  |  | — |  |  | — |  |  | 11.0 |  |  | 11.0 |  |  | — |  |  | — |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 7.0 |  |  | 7.0 |  |  | — |  |  | — |
Fixed income securities | 42.8 |  |  | 42.8 |  |  | — |  |  | — |  |  | 32.4 |  |  | 32.4 |  |  | — |  |  | — |
Total OPEB plan assets |  | $72.9 |  |  |  | $72.9 |  |  |  | $— |  |  |  | $— |  |  |  | $68.2 |  |  |  | $68.2 |  |  |  | $— |  |  |  | $— |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.6 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $1.3 |  |  |  | $— |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 5.5 |  |  | 5.5 |  |  | — |  |  | — |
Fixed income securities | 18.1 |  |  | 18.1 |  |  | — |  |  | — |  |  | 11.1 |  |  | 11.1 |  |  | — |  |  | — |
Total OPEB plan assets |  | $18.7 |  |  |  | $18.4 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $18.6 |  |  |  | $17.3 |  |  |  | $1.3 |  |  |  | $— |
For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016. 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016, respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  |  | 2016 |  |  | 2015 |  |  | 2017 |  | 2016 |  | 2015
401(k) costs |  | $24.8 |  |  |  | $23.6 |  |  |  | $24.9 |  |  |  | $12.8 |  |  |  | $12.0 |  |  |  | $12.7 |  |  |  | $11.1 |  |  |  | $10.7 |  |  |  | $11.2 |
(b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017, performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017, performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Compensation expense |  | $15.1 |  |  |  | $18.0 |  |  |  | $10.7 |  |  |  | $8.3 |  |  |  | $9.5 |  |  |  | $5.7 |  |  |  | $6.4 |  |  |  | $7.9 |  |  |  | $4.7 |
Income tax benefits | 6.2 |  |  | 7.4 |  |  | 4.4 |  |  | 3.4 |  |  | 4.0 |  |  | 2.4 |  |  | 2.6 |  |  | 3.2 |  |  | 1.9 |
As of December 31, 2017, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million, $3.1 million and $2.2 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three-year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Nonvested awards, January 1 | 257,599 |  |  | 288,430 |  |  | 288,848 |  |  | 93,320 |  |  | 116,412 |  |  | 127,330 |
Granted | 65,350 |  |  | 68,585 |  |  | 90,806 |  |  | 21,558 |  |  | 23,918 |  |  | 35,674 |
Vested | (99,438 | ) |  | (98,186 | ) |  | (91,224 | ) |  | (37,395 | ) |  | (42,760 | ) |  | (45,690 | )
Forfeited | — |  |  | (1,230 | ) |  | — |  |  | (5,746 | ) |  | (4,250 | ) |  | (902 | )
Nonvested awards, December 31 | 223,511 |  |  | 257,599 |  |  | 288,430 |  |  | 71,737 |  |  | 93,320 |  |  | 116,412 |
Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
| 2014 Grant |  | 2013 Grant |  | 2012 Grant |  | 2014 Grant |  | 2013 Grant |  | 2012 Grant
Performance awards vested | 99,438 |  | 98,186 |  | 91,224 |  | 37,395 |  | 42,760 |  | 45,690
Percentage of target number of performance awards | 147.5% |  | 165.0% |  | 167.5% |  | 147.5% |  | 165.0% |  | 167.5%
Aggregate payout value (in millions) | $5.6 |  | $5.1 |  | $5.1 |  | $1.5 |  | $1.7 |  | $1.6
Payout - cash (in millions) | $5.1 |  | $2.9 |  | $3.2 |  | $1.5 |  | $1.7 |  | $1.6
Payout - common stock shares issued | 5,185 |  | 22,408 |  | 21,950 |  | N/A |  | N/A |  | N/A
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017, by year of grant, were as follows:
| Performance Shares |  | Performance Units
| 2017 Grant |  | 2016 Grant |  | 2015 Grant |  | 2017 Grant |  | 2016 Grant |  | 2015 Grant
Nonvested awards at target | 65,350 |  |  | 67,355 |  |  | 90,806 |  |  | 18,600 |  |  | 21,227 |  |  | 31,910 |
Alliant Energy common stock closing price on December 29, 2017 |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  | N/A
Alliant Energy common stock closing price on grant date | N/A |  | N/A |  | N/A |  | N/A |  | N/A |  |  | $32.55 |
Estimated payout percentage based on performance criteria | 105 | % |  | 150 | % |  | 138 | % |  | 105 | % |  | 150 | % |  | 138 | %
Fair values of each nonvested award |  | $44.74 |  |  |  | $63.92 |  |  |  | $58.80 |  |  |  | $44.74 |  |  |  | $63.92 |  |  |  | $44.92 |
Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three-year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017, the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows:
| 2017 |  | 2016
| Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 67,355 |  |  |  | $33.96 |  |  | — |  |  |  | $— |
Granted | 65,350 |  |  | 39.12 |  |  | 68,585 |  |  | 33.96 |
Forfeited | — |  |  | — |  |  | (1,230 | ) |  | 33.90 |
Nonvested units, December 31 | 132,705 |  |  | 36.50 |  |  | 67,355 |  |  | 33.96 |
Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three-year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017, the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows:
| 2017 |  | 2016
Nonvested units, January 1 | 57,736 |  |  | — |
Granted | 56,013 |  |  | 58,790 |
Forfeited | — |  |  | (1,054 | )
Nonvested units, December 31 | 113,749 |  |  | 57,736 |
Performance-Contingent Restricted Stock and Performance-Contingent Cash Awards - As of December 31, 2017, the amount of nonvested performance-contingent restricted stock and performance-contingent cash awards was not material.
WPL [Member] |
Benefit Plans | BENEFIT PLANS (a) Pension and Other Postretirement Benefits Plans - Retirement benefits are provided to substantially all employees through various qualified and non-qualified non-contributory defined benefit pension plans (currently closed to new hires), and/or through defined contribution plans (including 401(k) savings plans). Benefits of the non-contributory defined benefit pension plans are based on the plan participant’s years of service, age and compensation. Benefits of the defined contribution plans are based on the plan participant’s years of service, age, compensation and contributions. Certain defined benefit postretirement health care and life benefits are provided to eligible retirees. In general, the retiree health care plans consist of fixed benefit subsidy structures and the retiree life insurance plans are non-contributory. IPL and WPL account for their participation in Alliant Energy and Corporate Services sponsored plans as multiple-employer plans. In IPL’s and WPL’s tables below, the defined benefit pension plan amounts represent those respective amounts for their bargaining unit employees covered under the qualified plans that they sponsor, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans. In IPL’s and WPL’s tables below, the OPEB plan amounts represent respective amounts for their employees, as well as amounts directly assigned to them related to their current and former non-bargaining employees who are participants in the Corporate Services sponsored OPEB plan. Assumptions - The assumptions for defined benefit pension and OPEB plans at the measurement date of December 31 were as follows:
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.66% |  | 4.19% |  | 4.47% |  | 3.53% |  | 3.98% |  | 4.30%
Discount rate for net periodic cost | 4.19% |  | 4.47% |  | 4.18% |  | 3.98% |  | 4.30% |  | 3.97%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 5.80% |  | 6.30% |  | 6.20%
Rate of compensation increase | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | 3.65 | % | - | 4.50% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.68% |  | 4.22% |  | 4.50% |  | 3.51% |  | 3.95% |  | 4.28%
Discount rate for net periodic cost | 4.22% |  | 4.50% |  | 4.20% |  | 3.95% |  | 4.28% |  | 3.94%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 6.20% |  | 6.60% |  | 6.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
| Qualified Defined Benefit Pension Plan |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Discount rate for benefit obligations | 3.69% |  | 4.23% |  | 4.51% |  | 3.51% |  | 3.96% |  | 4.28%
Discount rate for net periodic cost | 4.23% |  | 4.51% |  | 4.20% |  | 3.96% |  | 4.28% |  | 3.96%
Expected rate of return on plan assets | 7.60% |  | 7.60% |  | 7.60% |  | 3.50% |  | 4.70% |  | 4.60%
Rate of compensation increase | 3.65% |  | 3.65% |  | 3.65% |  | N/A |  | N/A |  | N/A
Medical cost trend on covered charges: |  |  |  |  |  |  |  |  |  |  |
Initial trend rate (end of year) | N/A |  | N/A |  | N/A |  | 6.75% |  | 7.00% |  | 7.25%
Ultimate trend rate | N/A |  | N/A |  | N/A |  | 5.00% |  | 5.00% |  | 5.00%
Expected rate of return on plan assets - The expected rate of return on plan assets is based on projected asset class returns using target allocations. A forward-looking building blocks approach is used, and historical returns, survey information and capital market information are analyzed to support the expected rate of return on plan assets assumption. Refer to “Investment Strategy for Plan Assets” below for additional information related to investment strategy and mix of assets for the pension and OPEB plans. Life Expectancy - The life expectancy assumption is used in determining the benefit obligation and net periodic benefit cost for defined benefit pension and OPEB plans. This assumption was updated to utilize mortality tables that were released in 2014 by the Society of Actuaries and updated in 2015 and 2016. Net Periodic Benefit Costs (Credits) - The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans are included in the tables below (in millions). Net periodic benefit costs are primarily included in “Other operation and maintenance” in the income statements.
Alliant Energy | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $12.5 |  |  |  | $12.6 |  |  |  | $15.9 |  |  |  | $5.0 |  |  |  | $5.3 |  |  |  | $5.5 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 53.6 |  |  | 8.6 |  |  | 9.4 |  |  | 9.1 |
Expected return on plan assets (a) | (65.5 | ) |  | (65.5 | ) |  | (75.0 | ) |  | (6.1 | ) |  | (6.1 | ) |  | (8.4 | )
Amortization of prior service credit (b) | (0.4 | ) |  | (0.3 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (4.1 | ) |  | (11.3 | )
Amortization of actuarial loss (c) | 37.6 |  |  | 37.4 |  |  | 35.4 |  |  | 3.8 |  |  | 4.7 |  |  | 4.8 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
Settlement losses (d) | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $36.1 |  |  |  | $37.2 |  |  |  | $30.2 |  |  |  | $11.1 |  |  |  | $9.2 |  |  |  | ($0.3 | )
IPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $7.3 |  |  |  | $7.5 |  |  |  | $8.8 |  |  |  | $2.1 |  |  |  | $2.3 |  |  |  | $2.4 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 25.0 |  |  | 3.5 |  |  | 3.8 |  |  | 3.8 |
Expected return on plan assets (a) | (30.8 | ) |  | (30.9 | ) |  | (35.8 | ) |  | (4.3 | ) |  | (4.3 | ) |  | (5.7 | )
Amortization of prior service credit (b) | (0.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | — |  |  | (2.6 | ) |  | (6.1 | )
Amortization of actuarial loss (c) | 16.1 |  |  | 16.5 |  |  | 15.3 |  |  | 2.0 |  |  | 2.6 |  |  | 2.3 |
|  | $15.9 |  |  |  | $17.4 |  |  |  | $13.2 |  |  |  | $3.3 |  |  |  | $1.8 |  |  |  | ($3.3 | )
WPL | Defined Benefit Pension Plans |  | OPEB Plans
2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Service cost |  | $4.9 |  |  |  | $4.9 |  |  |  | $5.8 |  |  |  | $1.9 |  |  |  | $2.0 |  |  |  | $2.1 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 22.6 |  |  | 3.4 |  |  | 3.8 |  |  | 3.7 |
Expected return on plan assets (a) | (28.5 | ) |  | (28.3 | ) |  | (32.4 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (1.5 | )
Amortization of prior service cost (credit) (b) | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (0.2 | ) |  | (0.9 | ) |  | (3.5 | )
Amortization of actuarial loss (c) | 18.5 |  |  | 17.6 |  |  | 16.8 |  |  | 1.6 |  |  | 1.8 |  |  | 2.2 |
Additional benefit costs | — |  |  | — |  |  | 0.5 |  |  | — |  |  | — |  |  | — |
|  | $16.8 |  |  |  | $16.7 |  |  |  | $13.5 |  |  |  | $5.9 |  |  |  | $5.9 |  |  |  | $3.0 |
(a) | The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
(b) | Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.
(c) | Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
(d) | Settlement losses related to payments made to retired executives of Alliant Energy.
The estimated amortization from “Regulatory assets” and “Regulatory liabilities” on the balance sheets and AOCL on Alliant Energy’s balance sheet into net periodic benefit cost in 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Defined Benefit |  |  |  | Defined Benefit |  |  |  | Defined Benefit |  |
| Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans |  | Pension Plans |  | OPEB Plans
Actuarial loss |  | $35.2 |  |  |  | $3.4 |  |  |  | $15.0 |  |  |  | $1.2 |  |  |  | $17.2 |  |  |  | $2.0 |
Prior service credit | (0.7 | ) |  | (0.2 | ) |  | (0.2 | ) |  | — |  |  | (0.2 | ) |  | (0.2 | )
|  | $34.5 |  |  |  | $3.2 |  |  |  | $14.8 |  |  |  | $1.2 |  |  |  | $17.0 |  |  |  | $1.8 |
Benefit Plan Assets and Obligations - A reconciliation of the funded status of qualified and non-qualified defined benefit pension and OPEB plans to the amounts recognized on the balance sheets at December 31 was as follows (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $1,244.3 |  |  |  | $1,206.3 |  |  |  | $220.1 |  |  |  | $221.4 |
Service cost | 12.5 |  |  | 12.6 |  |  | 5.0 |  |  | 5.3 |
Interest cost | 51.0 |  |  | 53.0 |  |  | 8.6 |  |  | 9.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Actuarial (gain) loss | 83.6 |  |  | 48.3 |  |  | 5.4 |  |  | (0.3 | )
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Net benefit obligation at December 31 | 1,303.1 |  |  | 1,244.3 |  |  | 222.3 |  |  | 220.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 895.7 |  |  | 895.0 |  |  | 105.8 |  |  | 106.9 |
Actual return on plan assets | 136.7 |  |  | 74.3 |  |  | 12.9 |  |  | 8.2 |
Employer contributions | 6.6 |  |  | 2.3 |  |  | 9.2 |  |  | 6.4 |
Plan participants’ contributions | — |  |  | — |  |  | 2.9 |  |  | 2.4 |
Gross benefits paid | (88.3 | ) |  | (75.9 | ) |  | (19.7 | ) |  | (18.1 | )
Fair value of plan assets at December 31 | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Under funded status at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $8.8 |  |  |  | $3.2 |
Current liabilities | (2.2 | ) |  | (6.5 | ) |  | (9.1 | ) |  | (8.6 | )
Pension and other benefit obligations | (350.2 | ) |  | (342.1 | ) |  | (110.9 | ) |  | (108.9 | )
Net amounts recognized at December 31 |  | ($352.4 | ) |  |  | ($348.6 | ) |  |  | ($111.2 | ) |  |  | ($114.3 | )
Amounts recognized in Regulatory Assets (refer to Note 2 for details) and AOCL (refer to Alliant Energy’s common equity statements for details) consist of: |  |  |  |  |  |  |
Net actuarial loss |  | $509.1 |  |  |  | $535.1 |  |  |  | $47.4 |  |  |  | $52.6 |
Prior service credit | (6.5 | ) |  | (6.9 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $502.6 |  |  |  | $528.2 |  |  |  | $46.1 |  |  |  | $51.1 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $570.4 |  |  |  | $556.1 |  |  |  | $90.1 |  |  |  | $91.3 |
Service cost | 7.3 |  |  | 7.5 |  |  | 2.1 |  |  | 2.3 |
Interest cost | 23.5 |  |  | 24.5 |  |  | 3.5 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Actuarial (gain) loss | 34.9 |  |  | 19.1 |  |  | (0.1 | ) |  | (0.7 | )
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Net benefit obligation at December 31 | 592.9 |  |  | 570.4 |  |  | 89.4 |  |  | 90.1 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 422.0 |  |  | 422.7 |  |  | 68.2 |  |  | 69.2 |
Actual return on plan assets | 64.3 |  |  | 35.3 |  |  | 8.9 |  |  | 5.3 |
Employer contributions | 0.6 |  |  | 0.8 |  |  | 2.0 |  |  | 0.3 |
Plan participants’ contributions | — |  |  | — |  |  | 1.0 |  |  | 0.9 |
Gross benefits paid | (43.2 | ) |  | (36.8 | ) |  | (7.2 | ) |  | (7.5 | )
Fair value of plan assets at December 31 | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Under funded status at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $5.9 |  |  |  | $0.4 |
Current liabilities | (0.5 | ) |  | (0.7 | ) |  | (2.0 | ) |  | (1.9 | )
Pension and other benefit obligations | (148.7 | ) |  | (147.7 | ) |  | (20.4 | ) |  | (20.4 | )
Net amounts recognized at December 31 |  | ($149.2 | ) |  |  | ($148.4 | ) |  |  | ($16.5 | ) |  |  | ($21.9 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $218.9 |  |  |  | $233.6 |  |  |  | $18.7 |  |  |  | $25.4 |
Prior service credit | (2.1 | ) |  | (2.3 | ) |  | — |  |  | — |
|  | $216.8 |  |  |  | $231.3 |  |  |  | $18.7 |  |  |  | $25.4 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Change in benefit obligation: |  |  |  |  |  |  |
Net benefit obligation at January 1 |  | $529.2 |  |  |  | $505.9 |  |  |  | $88.9 |  |  |  | $89.7 |
Service cost | 4.9 |  |  | 4.9 |  |  | 1.9 |  |  | 2.0 |
Interest cost | 21.8 |  |  | 22.3 |  |  | 3.4 |  |  | 3.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Actuarial loss | 38.3 |  |  | 25.7 |  |  | 4.1 |  |  | 0.5 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Net benefit obligation at December 31 | 559.8 |  |  | 529.2 |  |  | 90.4 |  |  | 88.9 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 389.7 |  |  | 386.8 |  |  | 18.6 |  |  | 18.7 |
Actual return on plan assets | 59.6 |  |  | 32.4 |  |  | 1.2 |  |  | 1.2 |
Employer contributions | 0.1 |  |  | 0.1 |  |  | 6.8 |  |  | 5.8 |
Plan participants’ contributions | — |  |  | — |  |  | 1.4 |  |  | 1.2 |
Gross benefits paid | (34.4 | ) |  | (29.6 | ) |  | (9.3 | ) |  | (8.3 | )
Fair value of plan assets at December 31 | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Under funded status at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Amounts recognized on the balance sheets consist of: |  |  |  |  |  |  |
Non-current assets |  | $— |  |  |  | $— |  |  |  | $2.9 |  |  |  | $2.7 |
Current liabilities | (0.1 | ) |  | (0.1 | ) |  | (6.8 | ) |  | (6.4 | )
Pension and other benefit obligations | (144.7 | ) |  | (139.4 | ) |  | (67.8 | ) |  | (66.6 | )
Net amounts recognized at December 31 |  | ($144.8 | ) |  |  | ($139.5 | ) |  |  | ($71.7 | ) |  |  | ($70.3 | )
Amounts recognized in Regulatory Assets consist of (refer to Note 2 for details): |  |  |  |  |  |  |
Net actuarial loss |  | $224.7 |  |  |  | $236.1 |  |  |  | $23.6 |  |  |  | $21.5 |
Prior service credit | (1.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (1.5 | )
|  | $223.2 |  |  |  | $234.7 |  |  |  | $22.3 |  |  |  | $20.0 |
Included in the following tables are accumulated benefit obligations, aggregate amounts applicable to defined benefit pension and OPEB plans with accumulated benefit obligations in excess of plan assets, as well as defined benefit pension plans with projected benefit obligations in excess of plan assets as of the December 31 measurement date (in millions):
| Defined Benefit Pension Plans |  | OPEB Plans
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $1,269.0 |  |  |  | $1,201.5 |  |  |  | $222.3 |  |  |  | $220.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 1,269.0 |  |  | 1,201.5 |  |  | 222.3 |  |  | 220.1 |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | 111.1 |  |  | 105.8 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 1,303.1 |  |  | 1,244.3 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 950.7 |  |  | 895.7 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $573.1 |  |  |  | $546.7 |  |  |  | $89.4 |  |  |  | $90.1 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 573.1 |  |  | 546.7 |  |  | 89.4 |  |  | 90.1 |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | 72.9 |  |  | 68.2 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 592.9 |  |  | 570.4 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 443.7 |  |  | 422.0 |  |  | N/A |  |  | N/A |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Accumulated benefit obligations |  | $548.1 |  |  |  | $513.2 |  |  |  | $90.4 |  |  |  | $88.9 |
Plans with accumulated benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Accumulated benefit obligations | 548.1 |  |  | 513.2 |  |  | 90.4 |  |  | 88.9 |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | 18.7 |  |  | 18.6 |
Plans with projected benefit obligations in excess of plan assets: |  |  |  |  |  |  |
Projected benefit obligations | 559.8 |  |  | 529.2 |  |  | N/A |  |  | N/A |
Fair value of plan assets | 415.0 |  |  | 389.7 |  |  | N/A |  |  | N/A |
In addition to the amounts recognized in regulatory assets in the above tables for IPL and WPL, regulatory assets were recognized for amounts associated with Corporate Services employees participating in other Alliant Energy sponsored benefit plans that were allocated to IPL and WPL at December 31 as follows (in millions):
| IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016
Regulatory assets |  | $38.9 |  |  |  | $37.3 |  |  |  | $28.1 |  |  |  | $30.0 |
Estimated Future Employer Contributions and Benefit Payments - Estimated funding for the qualified and non-qualified defined benefit pension and OPEB plans for 2018 is as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $6.3 |  |  |  | $4.4 |  |  |  | $0.3 |
OPEB plans | 9.0 |  |  | 2.0 |  |  | 6.8 |
(a) | Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.
Expected benefit payments for the qualified and non-qualified defined benefit plans, which reflect expected future service, as appropriate, are as follows (in millions):
Alliant Energy | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $72.2 |  |  |  | $73.9 |  |  |  | $76.4 |  |  |  | $77.1 |  |  |  | $92.9 |  |  |  | $398.2 |
OPEB | 18.2 |  |  | 18.4 |  |  | 17.7 |  |  | 17.5 |  |  | 17.2 |  |  | 80.1 |
|  | $90.4 |  |  |  | $92.3 |  |  |  | $94.1 |  |  |  | $94.6 |  |  |  | $110.1 |  |  |  | $478.3 |
IPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $33.9 |  |  |  | $33.8 |  |  |  | $36.5 |  |  |  | $36.6 |  |  |  | $37.6 |  |  |  | $188.4 |
OPEB | 7.1 |  |  | 7.1 |  |  | 7.2 |  |  | 7.1 |  |  | 7.0 |  |  | 32.6 |
|  | $41.0 |  |  |  | $40.9 |  |  |  | $43.7 |  |  |  | $43.7 |  |  |  | $44.6 |  |  |  | $221.0 |
WPL | 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023 - 2027
Defined benefit pension benefits |  | $31.4 |  |  |  | $32.0 |  |  |  | $32.4 |  |  |  | $32.4 |  |  |  | $32.7 |  |  |  | $167.8 |
OPEB | 8.3 |  |  | 8.3 |  |  | 7.5 |  |  | 7.3 |  |  | 7.0 |  |  | 31.8 |
|  | $39.7 |  |  |  | $40.3 |  |  |  | $39.9 |  |  |  | $39.7 |  |  |  | $39.7 |  |  |  | $199.6 |
Investment Strategy for Plan Assets - Investment strategies for defined benefit pension and OPEB plan assets combine preservation of principal and prudent risk-taking to protect the integrity of plan assets, in order to meet the obligations to plan participants while minimizing benefit costs over the long term. Investment risk of plan assets is mitigated through diversification, including broad U.S. equity, international equity and fixed income exposure, and global asset and risk parity strategies. Global asset and risk parity strategies include investments in global equity, global debt, commodities and currencies. Defined Benefit Pension Plan Assets - The asset mix of defined benefit pension plans is governed by allocation targets. Historical performance results and future expectations suggest that equity securities will provide higher total investment returns than fixed income securities over a long-term investment horizon. Consistent with the goals of meeting obligations to plan participants and minimizing benefit costs over the long-term, the defined benefit pension plans have a long-term investment posture more heavily weighted toward equity holdings. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. An overlay management service is also used to help maintain target allocations and meet liquidity needs. The overlay manager is authorized to use derivative financial instruments to facilitate this service. For separately managed accounts, prohibited investments include, but may not be limited to, direct ownership of real estate, margin trading, oil and gas limited partnerships and securities of the managers’ firms or affiliate firms. At December 31, 2017, the current target ranges and actual allocations for the defined benefit pension plan assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 3%
Equity securities - U.S. | 11 | % | - | 41% |  | 24%
Equity securities - international | 14 | % | - | 34% |  | 23%
Global asset securities | 5 | % | - | 15% |  | 10%
Risk parity securities | 5 | % | - | 15% |  | 10%
Fixed income securities | 20 | % | - | 40% |  | 30%
Other Postretirement Benefits Plan Assets - OPEB plan assets are comprised of specific assets within certain defined benefit pension plans (401(h) assets) as well as assets held in VEBA trusts. The investment strategy of the Corporate Services 401(h) assets mirrors those of the defined benefit pension plans, which are discussed above. For VEBA trusts with assets greater than $5 million and the WPL 401(h) assets, the mix among asset classes is governed by allocation targets. The asset allocation is monitored regularly and appropriate steps are taken as needed to rebalance the assets within the prescribed ranges. Mutual funds are used to achieve the desired diversification. At December 31, 2017, the current target ranges and actual allocations for VEBA trusts with assets greater than $5 million and the WPL 401(h) assets were as follows:
| Target Range |  | Actual
| Allocation |  | Allocation
Cash and equivalents | 0 | % | - | 5% |  | 1%
Equity securities - U.S. | 0 | % | - | 50% |  | 26%
Equity securities - international | 0 | % | - | 34% |  | 10%
Fixed income securities | 20 | % | - | 100% |  | 63%
Fair Value Measurements - Fair value measurement accounting establishes three levels of fair value hierarchy that prioritize the inputs to valuation techniques used to measure fair value. Refer to Note 14 for discussion of levels within the fair value hierarchy. Level 1 items include investments in securities held in registered investment companies, treasury bills and directly held equity securities, which are valued at the closing price reported in the active market in which the securities are traded. Level 2 items include fixed income securities consisting of corporate and government bonds and agency obligations, which are valued at the closing price reported in the active market for similar assets in which the individual securities are traded or based on yields currently available on comparable securities of issuers with similar credit ratings. Certain investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. These fair value amounts are included in the tables below to reconcile the fair value hierarchy to the respective total plan assets. At December 31, the fair values of qualified and non-qualified defined benefit pension plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $28.2 |  |  |  | $4.5 |  |  |  | $23.7 |  |  |  | $— |  |  |  | $30.4 |  |  |  | $5.0 |  |  |  | $25.4 |  |  |  | $— |
Equity securities - U.S. | 158.3 |  |  | 158.3 |  |  | — |  |  | — |  |  | 183.6 |  |  | 183.6 |  |  | — |  |  | — |
Equity securities - international | 137.5 |  |  | 137.5 |  |  | — |  |  | — |  |  | 97.4 |  |  | 97.4 |  |  | — |  |  | — |
Global asset securities | 49.4 |  |  | 49.4 |  |  | — |  |  | — |  |  | 53.0 |  |  | 53.0 |  |  | — |  |  | — |
Fixed income securities | 135.9 |  |  | 55.8 |  |  | 80.1 |  |  | — |  |  | 125.4 |  |  | 53.6 |  |  | 71.8 |  |  | — |
Total assets in fair value hierarchy | 509.3 |  |  |  | $405.5 |  |  |  | $103.8 |  |  |  | $— |  |  | 489.8 |  |  |  | $392.6 |  |  |  | $97.2 |  |  |  | $— |
Assets measured at net asset value | 441.1 |  |  |  |  |  |  |  |  | 405.9 |  |  |  |  |  |  |
Accrued investment income | 1.0 |  |  |  |  |  |  |  |  | 1.1 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.7 | ) |  |  |  |  |  |  |  | (1.1 | ) |  |  |  |  |  |
Total pension plan assets |  | $950.7 |  |  |  |  |  |  |  |  |  | $895.7 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $13.2 |  |  |  | $2.2 |  |  |  | $11.0 |  |  |  | $— |  |  |  | $14.4 |  |  |  | $2.4 |  |  |  | $12.0 |  |  |  | $— |
Equity securities - U.S. | 73.9 |  |  | 73.9 |  |  | — |  |  | — |  |  | 86.5 |  |  | 86.5 |  |  | — |  |  | — |
Equity securities - international | 64.2 |  |  | 64.2 |  |  | — |  |  | — |  |  | 45.9 |  |  | 45.9 |  |  | — |  |  | — |
Global asset securities | 23.0 |  |  | 23.0 |  |  | — |  |  | — |  |  | 24.9 |  |  | 24.9 |  |  | — |  |  | — |
Fixed income securities | 63.4 |  |  | 26.0 |  |  | 37.4 |  |  | — |  |  | 59.1 |  |  | 25.3 |  |  | 33.8 |  |  | — |
Total assets in fair value hierarchy | 237.7 |  |  |  | $189.3 |  |  |  | $48.4 |  |  |  | $— |  |  | 230.8 |  |  |  | $185.0 |  |  |  | $45.8 |  |  |  | $— |
Assets measured at net asset value | 205.8 |  |  |  |  |  |  |  |  | 191.2 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $443.7 |  |  |  |  |  |  |  |  |  | $422.0 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $12.3 |  |  |  | $2.0 |  |  |  | $10.3 |  |  |  | $— |  |  |  | $13.3 |  |  |  | $2.2 |  |  |  | $11.1 |  |  |  | $— |
Equity securities - U.S. | 69.1 |  |  | 69.1 |  |  | — |  |  | — |  |  | 79.9 |  |  | 79.9 |  |  | — |  |  | — |
Equity securities - international | 60.0 |  |  | 60.0 |  |  | — |  |  | — |  |  | 42.4 |  |  | 42.4 |  |  | — |  |  | — |
Global asset securities | 21.6 |  |  | 21.6 |  |  | — |  |  | — |  |  | 23.0 |  |  | 23.0 |  |  | — |  |  | — |
Fixed income securities | 59.3 |  |  | 24.3 |  |  | 35.0 |  |  | — |  |  | 54.5 |  |  | 23.3 |  |  | 31.2 |  |  | — |
Total assets in fair value hierarchy | 222.3 |  |  |  | $177.0 |  |  |  | $45.3 |  |  |  | $— |  |  | 213.1 |  |  |  | $170.8 |  |  |  | $42.3 |  |  |  | $— |
Assets measured at net asset value | 192.5 |  |  |  |  |  |  |  |  | 176.6 |  |  |  |  |  |  |
Accrued investment income | 0.5 |  |  |  |  |  |  |  |  | 0.5 |  |  |  |  |  |  |
Due to brokers, net (pending trades with brokers) | (0.3 | ) |  |  |  |  |  |  |  | (0.5 | ) |  |  |  |  |  |
Total pension plan assets |  | $415.0 |  |  |  |  |  |  |  |  |  | $389.7 |  |  |  |  |  |  |
At December 31, the fair values of OPEB plan assets were as follows (in millions):
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
Alliant Energy | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $1.2 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $— |  |  |  | $3.5 |  |  |  | $2.0 |  |  |  | $1.5 |  |  |  | $— |
Equity securities - U.S. | 27.9 |  |  | 27.9 |  |  | — |  |  | — |  |  | 22.5 |  |  | 22.5 |  |  | — |  |  | — |
Equity securities - international | 11.4 |  |  | 11.4 |  |  | — |  |  | — |  |  | 13.5 |  |  | 13.5 |  |  | — |  |  | — |
Global asset securities | 0.4 |  |  | 0.4 |  |  | — |  |  | — |  |  | 16.5 |  |  | 16.5 |  |  | — |  |  | — |
Fixed income securities | 66.6 |  |  | 66.0 |  |  | 0.6 |  |  | — |  |  | 46.8 |  |  | 46.2 |  |  | 0.6 |  |  | — |
Total assets in fair value hierarchy | 107.5 |  |  |  | $106.4 |  |  |  | $1.1 |  |  |  | $— |  |  | 102.8 |  |  |  | $100.7 |  |  |  | $2.1 |  |  |  | $— |
Assets measured at net asset value | 3.6 |  |  |  |  |  |  |  |  | 3.0 |  |  |  |  |  |  |
Total OPEB plan assets |  | $111.1 |  |  |  |  |  |  |  |  |  | $105.8 |  |  |  |  |  |  |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
IPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $— |  |  |  | $0.8 |  |  |  | $0.8 |  |  |  | $— |  |  |  | $— |
Equity securities - U.S. | 22.3 |  |  | 22.3 |  |  | — |  |  | — |  |  | 17.0 |  |  | 17.0 |  |  | — |  |  | — |
Equity securities - international | 7.5 |  |  | 7.5 |  |  | — |  |  | — |  |  | 11.0 |  |  | 11.0 |  |  | — |  |  | — |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 7.0 |  |  | 7.0 |  |  | — |  |  | — |
Fixed income securities | 42.8 |  |  | 42.8 |  |  | — |  |  | — |  |  | 32.4 |  |  | 32.4 |  |  | — |  |  | — |
Total OPEB plan assets |  | $72.9 |  |  |  | $72.9 |  |  |  | $— |  |  |  | $— |  |  |  | $68.2 |  |  |  | $68.2 |  |  |  | $— |  |  |  | $— |
| 2017 |  | 2016
| Fair |  | Level |  | Level |  | Level |  | Fair |  | Level |  | Level |  | Level
WPL | Value |  | 1 |  | 2 |  | 3 |  | Value |  | 1 |  | 2 |  | 3
Cash and equivalents |  | $0.6 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $1.3 |  |  |  | $— |
Global asset securities | — |  |  | — |  |  | — |  |  | — |  |  | 5.5 |  |  | 5.5 |  |  | — |  |  | — |
Fixed income securities | 18.1 |  |  | 18.1 |  |  | — |  |  | — |  |  | 11.1 |  |  | 11.1 |  |  | — |  |  | — |
Total OPEB plan assets |  | $18.7 |  |  |  | $18.4 |  |  |  | $0.3 |  |  |  | $— |  |  |  | $18.6 |  |  |  | $17.3 |  |  |  | $1.3 |  |  |  | $— |
For the various defined benefit pension and OPEB plans, Alliant Energy common stock represented less than 1% of assets directly held in the plans at December 31, 2017 and 2016. 401(k) Savings Plans - A significant number of employees participate in defined contribution retirement plans (401(k) savings plans). Alliant Energy common stock directly held by participants represented 11.5% and 12.6% of total assets in the 401(k) savings plans at December 31, 2017 and 2016, respectively. Costs related to the 401(k) savings plans, which are partially based on the participants’ contributions and include allocated costs associated with Corporate Services employees for IPL and WPL, were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  |  | 2016 |  |  | 2015 |  |  | 2017 |  | 2016 |  | 2015
401(k) costs |  | $24.8 |  |  |  | $23.6 |  |  |  | $24.9 |  |  |  | $12.8 |  |  |  | $12.0 |  |  |  | $12.7 |  |  |  | $11.1 |  |  |  | $10.7 |  |  |  | $11.2 |
(b) Equity-based Compensation Plans - In 2015, Alliant Energy’s shareowners approved the Amended and Restated OIP, which permits the grant of shares of Alliant Energy common stock, restricted stock, restricted stock units, performance shares, performance units, and other stock-based or cash-based awards to key employees. At December 31, 2017, performance shares, performance-contingent restricted stock and restricted stock units (performance- and time-vesting) were outstanding under the Amended and Restated OIP, and 7.1 million shares of Alliant Energy’s common stock remained available for grants under the Amended and Restated OIP. Alliant Energy satisfies share payouts related to equity awards under the Amended and Restated OIP through the issuance of new shares of its common stock. Alliant Energy also has the DLIP, which permits the grant of cash-based long-term awards, including performance units, restricted cash awards and restricted units, to certain key employees. At December 31, 2017, performance units, performance-contingent cash awards and restricted units (performance- and time-vesting) were outstanding under the DLIP. There is no limit to the number of grants that can be made under the DLIP and Alliant Energy satisfies all payouts under the DLIP through cash payments. Nonvested awards generally do not have non-forfeitable rights to dividends or dividend equivalents when dividends are paid to common shareowners. A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Compensation expense |  | $15.1 |  |  |  | $18.0 |  |  |  | $10.7 |  |  |  | $8.3 |  |  |  | $9.5 |  |  |  | $5.7 |  |  |  | $6.4 |  |  |  | $7.9 |  |  |  | $4.7 |
Income tax benefits | 6.2 |  |  | 7.4 |  |  | 4.4 |  |  | 3.4 |  |  | 4.0 |  |  | 2.4 |  |  | 2.6 |  |  | 3.2 |  |  | 1.9 |
As of December 31, 2017, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $5.6 million, $3.1 million and $2.2 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Share-based compensation expense is recognized on a straight-line basis over the requisite service periods and is primarily recorded in “Other operation and maintenance” in the income statements. Performance Shares and Performance Units - Payouts of performance shares under the Amended and Restated OIP and performance units under the DLIP to key employees are contingent upon achievement over three-year periods of specified performance criteria, which currently include metrics of total shareowner return relative to an investor-owned utility peer group. Performance shares can be paid out in shares of Alliant Energy’s common stock, cash or a combination of cash and stock. Performance units must be paid out in cash. Alliant Energy assumes it will make future payouts of its performance shares and performance units in cash; therefore, performance shares and performance units are accounted for as liability awards. A summary of the performance shares and performance units activity, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Nonvested awards, January 1 | 257,599 |  |  | 288,430 |  |  | 288,848 |  |  | 93,320 |  |  | 116,412 |  |  | 127,330 |
Granted | 65,350 |  |  | 68,585 |  |  | 90,806 |  |  | 21,558 |  |  | 23,918 |  |  | 35,674 |
Vested | (99,438 | ) |  | (98,186 | ) |  | (91,224 | ) |  | (37,395 | ) |  | (42,760 | ) |  | (45,690 | )
Forfeited | — |  |  | (1,230 | ) |  | — |  |  | (5,746 | ) |  | (4,250 | ) |  | (902 | )
Nonvested awards, December 31 | 223,511 |  |  | 257,599 |  |  | 288,430 |  |  | 71,737 |  |  | 93,320 |  |  | 116,412 |
Granted Awards - Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2017 and 2016, the value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. For performance units granted in 2015, the value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance shares and performance units is dependent upon actual performance and may range from zero to 200% of the target number of awards. Compensation expense for performance shares and performance units is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Vested Awards - Certain performance shares and performance units vested, resulting in payouts (a combination of cash and common stock for the performance shares and cash only for the performance units) as follows:
| Performance Shares |  | Performance Units
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
| 2014 Grant |  | 2013 Grant |  | 2012 Grant |  | 2014 Grant |  | 2013 Grant |  | 2012 Grant
Performance awards vested | 99,438 |  | 98,186 |  | 91,224 |  | 37,395 |  | 42,760 |  | 45,690
Percentage of target number of performance awards | 147.5% |  | 165.0% |  | 167.5% |  | 147.5% |  | 165.0% |  | 167.5%
Aggregate payout value (in millions) | $5.6 |  | $5.1 |  | $5.1 |  | $1.5 |  | $1.7 |  | $1.6
Payout - cash (in millions) | $5.1 |  | $2.9 |  | $3.2 |  | $1.5 |  | $1.7 |  | $1.6
Payout - common stock shares issued | 5,185 |  | 22,408 |  | 21,950 |  | N/A |  | N/A |  | N/A
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at December 31, 2017, by year of grant, were as follows:
| Performance Shares |  | Performance Units
| 2017 Grant |  | 2016 Grant |  | 2015 Grant |  | 2017 Grant |  | 2016 Grant |  | 2015 Grant
Nonvested awards at target | 65,350 |  |  | 67,355 |  |  | 90,806 |  |  | 18,600 |  |  | 21,227 |  |  | 31,910 |
Alliant Energy common stock closing price on December 29, 2017 |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  |  | $42.61 |  |  | N/A
Alliant Energy common stock closing price on grant date | N/A |  | N/A |  | N/A |  | N/A |  | N/A |  |  | $32.55 |
Estimated payout percentage based on performance criteria | 105 | % |  | 150 | % |  | 138 | % |  | 105 | % |  | 150 | % |  | 138 | %
Fair values of each nonvested award |  | $44.74 |  |  |  | $63.92 |  |  |  | $58.80 |  |  |  | $44.74 |  |  |  | $63.92 |  |  |  | $44.92 |
Performance Restricted Stock Units and Performance Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as performance restricted stock units under the Amended and Restated OIP, and performance restricted units and key employee performance restricted units under the DLIP. Payouts of these units are based on the achievement of certain performance targets (currently specified growth of consolidated income from continuing operations) during a three-year performance period. The actual number of units that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of units. If performance targets are not met during the performance period, these units are forfeited. As of December 31, 2017, the amount of nonvested performance restricted units and key employee performance restricted units was not material. Performance Restricted Stock Units - Performance restricted stock units generally must be paid out in shares and are accounted for as equity awards. Each performance restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock on the grant date of the award. Compensation expense is recorded ratably over the performance period based on a probability assessment of payouts for the awards at each reporting period. A summary of the performance restricted stock units activity, with amounts representing the target number of units, was as follows:
| 2017 |  | 2016
| Units |  | Weighted Average Grant Date Fair Value |  | Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 67,355 |  |  |  | $33.96 |  |  | — |  |  |  | $— |
Granted | 65,350 |  |  | 39.12 |  |  | 68,585 |  |  | 33.96 |
Forfeited | — |  |  | — |  |  | (1,230 | ) |  | 33.90 |
Nonvested units, December 31 | 132,705 |  |  | 36.50 |  |  | 67,355 |  |  | 33.96 |
Restricted Stock Units and Restricted Units - Alliant Energy granted new types of share-based compensation awards to key employees beginning in 2016 referred to as restricted stock units under the Amended and Restated OIP and restricted units under the DLIP. Payouts of these units are based on the expiration of a three-year time-vesting period. Each restricted stock unit’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the time-vesting period. Compensation expense is recorded ratably over the performance period based on the fair value of the awards at each reporting period. Restricted stock units can be paid out in shares of Alliant Energy common stock, cash or a combination of cash and stock. Alliant Energy assumes it will make future payouts of its restricted stock units in cash; therefore, restricted stock units are accounted for as liability awards. As of December 31, 2017, the amount of nonvested restricted units was not material. A summary of the restricted stock units activity was as follows:
| 2017 |  | 2016
Nonvested units, January 1 | 57,736 |  |  | — |
Granted | 56,013 |  |  | 58,790 |
Forfeited | — |  |  | (1,054 | )
Nonvested units, December 31 | 113,749 |  |  | 57,736 |
Performance-Contingent Restricted Stock and Performance-Contingent Cash Awards - As of December 31, 2017, the amount of nonvested performance-contingent restricted stock and performance-contingent cash awards was not material.

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