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Benefit Plans | 9 Months Ended
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Sep. 30, 2018
Benefit Plans | BENEFIT PLANS NOTE 10(a) Pension and Other Postretirement Benefits Plans - Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. 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.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $3.1 |  |  |  | $3.1 |  |  |  | $9.1 |  |  |  | $9.3 |  |  |  | $1.0 |  |  |  | $1.2 |  |  |  | $3.1 |  |  |  | $3.7 |
Interest cost | 11.7 |  |  | 12.7 |  |  | 35.1 |  |  | 38.3 |  |  | 2.0 |  |  | 2.2 |  |  | 5.8 |  |  | 6.5 |
Expected return on plan assets | (17.4 | ) |  | (16.3 | ) |  | (52.3 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.5 | ) |  | (4.5 | ) |  | (4.6 | )
Amortization of prior service credit | (0.2 | ) |  | (0.1 | ) |  | (0.5 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 8.8 |  |  | 9.4 |  |  | 26.4 |  |  | 28.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 2.9 |
Settlement losses (a) | — |  |  | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $6.0 |  |  |  | $9.7 |  |  |  | $17.8 |  |  |  | $27.3 |  |  |  | $2.3 |  |  |  | $2.8 |  |  |  | $6.8 |  |  |  | $8.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.5 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $1.6 |
Interest cost | 5.3 |  |  | 5.9 |  |  | 16.0 |  |  | 17.6 |  |  | 0.7 |  |  | 0.8 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (8.1 | ) |  | (7.7 | ) |  | (24.4 | ) |  | (23.1 | ) |  | (1.1 | ) |  | (1.0 | ) |  | (3.3 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 11.2 |  |  | 12.1 |  |  | 0.4 |  |  | 0.5 |  |  | 1.0 |  |  | 1.5 |
|  | $2.7 |  |  |  | $4.0 |  |  |  | $8.2 |  |  |  | $12.0 |  |  |  | $0.4 |  |  |  | $0.8 |  |  |  | $1.3 |  |  |  | $2.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.1 |  |  |  | $1.2 |  |  |  | $3.3 |  |  |  | $3.6 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.2 |  |  |  | $1.4 |
Interest cost | 5.0 |  |  | 5.5 |  |  | 15.1 |  |  | 16.4 |  |  | 0.8 |  |  | 0.9 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (7.6 | ) |  | (7.2 | ) |  | (22.8 | ) |  | (21.4 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.5 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | 0.1 |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.3 |  |  | 4.6 |  |  | 12.9 |  |  | 13.9 |  |  | 0.5 |  |  | 0.4 |  |  | 1.5 |  |  | 1.2 |
|  | $2.8 |  |  |  | $4.2 |  |  |  | $8.4 |  |  |  | $12.6 |  |  |  | $1.5 |  |  |  | $1.5 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
NOTE 10(b) Equity-based Compensation Plans - A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Compensation expense |  | $4.2 |  |  |  | $5.1 |  |  |  | $12.6 |  |  |  | $9.9 |  |  |  | $2.4 |  |  |  | $2.8 |  |  |  | $7.0 |  |  |  | $5.4 |  |  |  | $1.7 |  |  |  | $2.1 |  |  |  | $5.1 |  |  |  | $4.1 |
Income tax benefits | 1.2 |  |  | 2.1 |  |  | 3.6 |  |  | 4.0 |  |  | 0.7 |  |  | 1.1 |  |  | 2.1 |  |  | 2.2 |  |  | 0.5 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
As of September 30, 2018, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $8.3 million, $4.6 million and $3.5 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Performance Shares and Performance Units - A summary of the performance shares and performance units activity for the nine months ended September 30, 2018, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
Nonvested awards, January 1 | 223,511 |  |  | 71,737 |
Granted | 74,163 |  |  | 19,840 |
Vested | (90,806 | ) |  | (31,910 | )
Forfeited | (905 | ) |  | (1,906 | )
Nonvested awards, September 30 | 205,963 |  |  | 57,761 |
Vested Awards - During the nine months ended September 30, 2018, certain performance shares and performance units that were granted in 2015 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
Performance awards vested | 90,806 |  |  | 31,910 |
Percentage of target number of performance awards | 137.5 | % |  | 137.5 | %
Aggregate payout value (in millions) |  | $5.3 |  |  |  | $1.4 |
Payout - cash (in millions) |  | $4.9 |  |  |  | $1.4 |
Payout - common stock shares issued | 5,078 |  |  | N/A |
Fair Value of Awards - At September 28, 2018, Alliant Energy’s common stock closing price was $42.57. Additional information related to fair values of nonvested performance shares and performance units at September 30, 2018, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2018 Grant |  | 2017 Grant |  | 2016 Grant |  | 2018 Grant |  | 2017 Grant |  | 2016 Grant
Nonvested awards at target | 73,258 |  |  | 65,350 |  |  | 67,355 |  |  | 19,196 |  |  | 18,062 |  |  | 20,503 |
Estimated payout percentage based on performance criteria | 85 | % |  | 105 | % |  | 143 | % |  | 85 | % |  | 105 | % |  | 143 | %
Fair values of each nonvested award |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |  |  |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |
Performance Restricted Stock Units - A summary of the performance restricted stock units activity for the nine months ended September 30, 2018, with amounts representing the target number of units, was as follows:
| Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 132,705 |  |  |  | $36.50 |
Granted | 74,163 |  |  | 38.60 |
Forfeited | (905 | ) |  | 38.60 |
Nonvested units, September 30 | 205,963 |  |  | 37.25 |
Restricted Stock Units - A summary of the restricted stock units activity for the nine months ended September 30, 2018, was as follows:
Nonvested units, January 1 | 113,749 |
Granted | 63,568 |
Forfeited | (775 | )
Nonvested units, September 30 | 176,542 |
IPL [Member] |
Benefit Plans | BENEFIT PLANS NOTE 10(a) Pension and Other Postretirement Benefits Plans - Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. 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.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $3.1 |  |  |  | $3.1 |  |  |  | $9.1 |  |  |  | $9.3 |  |  |  | $1.0 |  |  |  | $1.2 |  |  |  | $3.1 |  |  |  | $3.7 |
Interest cost | 11.7 |  |  | 12.7 |  |  | 35.1 |  |  | 38.3 |  |  | 2.0 |  |  | 2.2 |  |  | 5.8 |  |  | 6.5 |
Expected return on plan assets | (17.4 | ) |  | (16.3 | ) |  | (52.3 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.5 | ) |  | (4.5 | ) |  | (4.6 | )
Amortization of prior service credit | (0.2 | ) |  | (0.1 | ) |  | (0.5 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 8.8 |  |  | 9.4 |  |  | 26.4 |  |  | 28.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 2.9 |
Settlement losses (a) | — |  |  | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $6.0 |  |  |  | $9.7 |  |  |  | $17.8 |  |  |  | $27.3 |  |  |  | $2.3 |  |  |  | $2.8 |  |  |  | $6.8 |  |  |  | $8.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.5 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $1.6 |
Interest cost | 5.3 |  |  | 5.9 |  |  | 16.0 |  |  | 17.6 |  |  | 0.7 |  |  | 0.8 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (8.1 | ) |  | (7.7 | ) |  | (24.4 | ) |  | (23.1 | ) |  | (1.1 | ) |  | (1.0 | ) |  | (3.3 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 11.2 |  |  | 12.1 |  |  | 0.4 |  |  | 0.5 |  |  | 1.0 |  |  | 1.5 |
|  | $2.7 |  |  |  | $4.0 |  |  |  | $8.2 |  |  |  | $12.0 |  |  |  | $0.4 |  |  |  | $0.8 |  |  |  | $1.3 |  |  |  | $2.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.1 |  |  |  | $1.2 |  |  |  | $3.3 |  |  |  | $3.6 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.2 |  |  |  | $1.4 |
Interest cost | 5.0 |  |  | 5.5 |  |  | 15.1 |  |  | 16.4 |  |  | 0.8 |  |  | 0.9 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (7.6 | ) |  | (7.2 | ) |  | (22.8 | ) |  | (21.4 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.5 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | 0.1 |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.3 |  |  | 4.6 |  |  | 12.9 |  |  | 13.9 |  |  | 0.5 |  |  | 0.4 |  |  | 1.5 |  |  | 1.2 |
|  | $2.8 |  |  |  | $4.2 |  |  |  | $8.4 |  |  |  | $12.6 |  |  |  | $1.5 |  |  |  | $1.5 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
NOTE 10(b) Equity-based Compensation Plans - A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Compensation expense |  | $4.2 |  |  |  | $5.1 |  |  |  | $12.6 |  |  |  | $9.9 |  |  |  | $2.4 |  |  |  | $2.8 |  |  |  | $7.0 |  |  |  | $5.4 |  |  |  | $1.7 |  |  |  | $2.1 |  |  |  | $5.1 |  |  |  | $4.1 |
Income tax benefits | 1.2 |  |  | 2.1 |  |  | 3.6 |  |  | 4.0 |  |  | 0.7 |  |  | 1.1 |  |  | 2.1 |  |  | 2.2 |  |  | 0.5 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
As of September 30, 2018, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $8.3 million, $4.6 million and $3.5 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Performance Shares and Performance Units - A summary of the performance shares and performance units activity for the nine months ended September 30, 2018, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
Nonvested awards, January 1 | 223,511 |  |  | 71,737 |
Granted | 74,163 |  |  | 19,840 |
Vested | (90,806 | ) |  | (31,910 | )
Forfeited | (905 | ) |  | (1,906 | )
Nonvested awards, September 30 | 205,963 |  |  | 57,761 |
Vested Awards - During the nine months ended September 30, 2018, certain performance shares and performance units that were granted in 2015 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
Performance awards vested | 90,806 |  |  | 31,910 |
Percentage of target number of performance awards | 137.5 | % |  | 137.5 | %
Aggregate payout value (in millions) |  | $5.3 |  |  |  | $1.4 |
Payout - cash (in millions) |  | $4.9 |  |  |  | $1.4 |
Payout - common stock shares issued | 5,078 |  |  | N/A |
Fair Value of Awards - At September 28, 2018, Alliant Energy’s common stock closing price was $42.57. Additional information related to fair values of nonvested performance shares and performance units at September 30, 2018, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2018 Grant |  | 2017 Grant |  | 2016 Grant |  | 2018 Grant |  | 2017 Grant |  | 2016 Grant
Nonvested awards at target | 73,258 |  |  | 65,350 |  |  | 67,355 |  |  | 19,196 |  |  | 18,062 |  |  | 20,503 |
Estimated payout percentage based on performance criteria | 85 | % |  | 105 | % |  | 143 | % |  | 85 | % |  | 105 | % |  | 143 | %
Fair values of each nonvested award |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |  |  |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |
Performance Restricted Stock Units - A summary of the performance restricted stock units activity for the nine months ended September 30, 2018, with amounts representing the target number of units, was as follows:
| Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 132,705 |  |  |  | $36.50 |
Granted | 74,163 |  |  | 38.60 |
Forfeited | (905 | ) |  | 38.60 |
Nonvested units, September 30 | 205,963 |  |  | 37.25 |
Restricted Stock Units - A summary of the restricted stock units activity for the nine months ended September 30, 2018, was as follows:
Nonvested units, January 1 | 113,749 |
Granted | 63,568 |
Forfeited | (775 | )
Nonvested units, September 30 | 176,542 |
WPL [Member] |
Benefit Plans | BENEFIT PLANS NOTE 10(a) Pension and Other Postretirement Benefits Plans - Net Periodic Benefit Costs - The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three and nine months ended September 30 are included in the tables below (in millions). The service cost component of net periodic benefit costs is included in “Other operation and maintenance” expenses in the income statements and all other components of net periodic benefit costs are included in “Other (income) and deductions” in the income statements. 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.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
Alliant Energy | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $3.1 |  |  |  | $3.1 |  |  |  | $9.1 |  |  |  | $9.3 |  |  |  | $1.0 |  |  |  | $1.2 |  |  |  | $3.1 |  |  |  | $3.7 |
Interest cost | 11.7 |  |  | 12.7 |  |  | 35.1 |  |  | 38.3 |  |  | 2.0 |  |  | 2.2 |  |  | 5.8 |  |  | 6.5 |
Expected return on plan assets | (17.4 | ) |  | (16.3 | ) |  | (52.3 | ) |  | (49.1 | ) |  | (1.5 | ) |  | (1.5 | ) |  | (4.5 | ) |  | (4.6 | )
Amortization of prior service credit | (0.2 | ) |  | (0.1 | ) |  | (0.5 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 8.8 |  |  | 9.4 |  |  | 26.4 |  |  | 28.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 2.9 |
Settlement losses (a) | — |  |  | 0.9 |  |  | — |  |  | 0.9 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $6.0 |  |  |  | $9.7 |  |  |  | $17.8 |  |  |  | $27.3 |  |  |  | $2.3 |  |  |  | $2.8 |  |  |  | $6.8 |  |  |  | $8.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.8 |  |  |  | $1.8 |  |  |  | $5.5 |  |  |  | $5.5 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $1.6 |
Interest cost | 5.3 |  |  | 5.9 |  |  | 16.0 |  |  | 17.6 |  |  | 0.7 |  |  | 0.8 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (8.1 | ) |  | (7.7 | ) |  | (24.4 | ) |  | (23.1 | ) |  | (1.1 | ) |  | (1.0 | ) |  | (3.3 | ) |  | (3.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 11.2 |  |  | 12.1 |  |  | 0.4 |  |  | 0.5 |  |  | 1.0 |  |  | 1.5 |
|  | $2.7 |  |  |  | $4.0 |  |  |  | $8.2 |  |  |  | $12.0 |  |  |  | $0.4 |  |  |  | $0.8 |  |  |  | $1.3 |  |  |  | $2.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Service cost |  | $1.1 |  |  |  | $1.2 |  |  |  | $3.3 |  |  |  | $3.6 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $1.2 |  |  |  | $1.4 |
Interest cost | 5.0 |  |  | 5.5 |  |  | 15.1 |  |  | 16.4 |  |  | 0.8 |  |  | 0.9 |  |  | 2.3 |  |  | 2.6 |
Expected return on plan assets | (7.6 | ) |  | (7.2 | ) |  | (22.8 | ) |  | (21.4 | ) |  | (0.2 | ) |  | (0.2 | ) |  | (0.5 | ) |  | (0.6 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | 0.1 |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.3 |  |  | 4.6 |  |  | 12.9 |  |  | 13.9 |  |  | 0.5 |  |  | 0.4 |  |  | 1.5 |  |  | 1.2 |
|  | $2.8 |  |  |  | $4.2 |  |  |  | $8.4 |  |  |  | $12.6 |  |  |  | $1.5 |  |  |  | $1.5 |  |  |  | $4.4 |  |  |  | $4.4 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
NOTE 10(b) Equity-based Compensation Plans - A summary of compensation expense, including amounts allocated to IPL and WPL, and the related income tax benefits recognized for share-based compensation awards for the three and nine months ended September 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Compensation expense |  | $4.2 |  |  |  | $5.1 |  |  |  | $12.6 |  |  |  | $9.9 |  |  |  | $2.4 |  |  |  | $2.8 |  |  |  | $7.0 |  |  |  | $5.4 |  |  |  | $1.7 |  |  |  | $2.1 |  |  |  | $5.1 |  |  |  | $4.1 |
Income tax benefits | 1.2 |  |  | 2.1 |  |  | 3.6 |  |  | 4.0 |  |  | 0.7 |  |  | 1.1 |  |  | 2.1 |  |  | 2.2 |  |  | 0.5 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
As of September 30, 2018, Alliant Energy’s, IPL’s and WPL’s total unrecognized compensation cost related to share-based compensation awards was $8.3 million, $4.6 million and $3.5 million, respectively, which is expected to be recognized over a weighted average period of between one and two years. Performance Shares and Performance Units - A summary of the performance shares and performance units activity for the nine months ended September 30, 2018, with amounts representing the target number of awards, was as follows:
| Performance Shares |  | Performance Units
Nonvested awards, January 1 | 223,511 |  |  | 71,737 |
Granted | 74,163 |  |  | 19,840 |
Vested | (90,806 | ) |  | (31,910 | )
Forfeited | (905 | ) |  | (1,906 | )
Nonvested awards, September 30 | 205,963 |  |  | 57,761 |
Vested Awards - During the nine months ended September 30, 2018, certain performance shares and performance units that were granted in 2015 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
Performance awards vested | 90,806 |  |  | 31,910 |
Percentage of target number of performance awards | 137.5 | % |  | 137.5 | %
Aggregate payout value (in millions) |  | $5.3 |  |  |  | $1.4 |
Payout - cash (in millions) |  | $4.9 |  |  |  | $1.4 |
Payout - common stock shares issued | 5,078 |  |  | N/A |
Fair Value of Awards - At September 28, 2018, Alliant Energy’s common stock closing price was $42.57. Additional information related to fair values of nonvested performance shares and performance units at September 30, 2018, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2018 Grant |  | 2017 Grant |  | 2016 Grant |  | 2018 Grant |  | 2017 Grant |  | 2016 Grant
Nonvested awards at target | 73,258 |  |  | 65,350 |  |  | 67,355 |  |  | 19,196 |  |  | 18,062 |  |  | 20,503 |
Estimated payout percentage based on performance criteria | 85 | % |  | 105 | % |  | 143 | % |  | 85 | % |  | 105 | % |  | 143 | %
Fair values of each nonvested award |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |  |  |  | $36.18 |  |  |  | $44.70 |  |  |  | $60.88 |
Performance Restricted Stock Units - A summary of the performance restricted stock units activity for the nine months ended September 30, 2018, with amounts representing the target number of units, was as follows:
| Units |  | Weighted Average Grant Date Fair Value
Nonvested units, January 1 | 132,705 |  |  |  | $36.50 |
Granted | 74,163 |  |  | 38.60 |
Forfeited | (905 | ) |  | 38.60 |
Nonvested units, September 30 | 205,963 |  |  | 37.25 |
Restricted Stock Units - A summary of the restricted stock units activity for the nine months ended September 30, 2018, was as follows:
Nonvested units, January 1 | 113,749 |
Granted | 63,568 |
Forfeited | (775 | )
Nonvested units, September 30 | 176,542 |

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