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Benefit Plans (Tables) | 3 Months Ended
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Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three months ended March 31 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans 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 plans 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
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $1.2 |  |  |  | $1.3 |
Interest cost | 12.8 |  |  | 13.3 |  |  | 2.2 |  |  | 2.3 |
Expected return on plan assets | (16.4 | ) |  | (16.4 | ) |  | (1.5 | ) |  | (1.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (1.0 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 1.0 |  |  | 1.2 |
|  | $8.8 |  |  |  | $9.3 |  |  |  | $2.8 |  |  |  | $2.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.9 |  |  |  | $0.5 |  |  |  | $0.6 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 0.9 |  |  | 1.0 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service credit | — |  |  | — |  |  | — |  |  | (0.7 | )
Amortization of actuarial loss | 4.0 |  |  | 4.1 |  |  | 0.5 |  |  | 0.6 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $0.8 |  |  |  | $0.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.2 |  |  |  | $0.5 |  |  |  | $0.5 |
Interest cost | 5.5 |  |  | 5.6 |  |  | 0.9 |  |  | 0.9 |
Expected return on plan assets | (7.1 | ) |  | (7.1 | ) |  | (0.2 | ) |  | (0.2 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 0.4 |  |  | 0.5 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $1.5 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three months ended March 31, costs related to the 401(k) savings plan, 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 |  | 2017 |  | 2016 |  | 2017 |  | 2016
401(k) costs |  | $6.5 |  |  |  | $6.2 |  |  |  | $3.4 |  |  |  | $3.1 |  |  |  | $2.9 |  |  |  | $2.8 |
Recognized Compensation Expense And Income Tax Benefits | 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 months ended March 31 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $3.2 |  |  |  | $5.3 |  |  |  | $1.7 |  |  |  | $2.8 |  |  |  | $1.4 |  |  |  | $2.3 |
Income tax benefits | 1.3 |  |  | 2.2 |  |  | 0.7 |  |  | 1.1 |  |  | 0.5 |  |  | 0.9 |
Performance Shares and Performance Units [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | 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 |  | 2017 |  | 2016
Nonvested awards, January 1 | 257,599 |  |  | 288,430 |  |  | 93,320 |  |  | 116,412 |
Granted | 63,804 |  |  | 67,552 |  |  | 21,558 |  |  | 23,918 |
Vested | (99,438 | ) |  | (98,186 | ) |  | (37,395 | ) |  | (42,760 | )
Forfeited | — |  |  | (1,230 | ) |  | (497 | ) |  | (764 | )
Nonvested awards, March 31 | 221,965 |  |  | 256,566 |  |  | 76,986 |  |  | 96,806 |
Vested Awards - During the three months ended March 31, 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 |  | 2017 |  | 2016
| 2014 Grant |  | 2013 Grant |  | 2014 Grant |  | 2013 Grant
Performance awards vested | 99,438 |  |  | 98,186 |  |  | 37,395 |  |  | 42,760 |
Percentage of target number of performance awards | 147.5 | % |  | 165.0 | % |  | 147.5 | % |  | 165.0 | %
Aggregate payout value (in millions) |  | $5.6 |  |  |  | $5.1 |  |  |  | $1.5 |  |  |  | $1.7 |
Payout - cash (in millions) |  | $5.1 |  |  |  | $2.9 |  |  |  | $1.5 |  |  |  | $1.7 |
Payout - common stock shares issued | 5,185 |  |  | 22,408 |  |  | N/A |  | N/A
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at March 31, 2017, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2017 Grant |  | 2016 Grant |  | 2015 Grant |  | 2017 Grant |  | 2016 Grant |  | 2015 Grant
Nonvested awards at target | 63,804 |  |  | 67,355 |  |  | 90,806 |  |  | 21,061 |  |  | 22,657 |  |  | 33,268 |
Alliant Energy common stock closing price on March 31, 2017 |  | $39.61 |  |  |  | $39.61 |  |  |  | $39.61 |  |  |  | $39.61 |  |  |  | $39.61 |  |  | 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 | 100 | % |  | 160 | % |  | 130 | % |  | 100 | % |  | 160 | % |  | 130 | %
Fair values of each nonvested award |  | $39.61 |  |  |  | $63.38 |  |  |  | $51.49 |  |  |  | $39.61 |  |  |  | $63.38 |  |  |  | $42.32 |
Performance Restricted Stock Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | 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 | 63,804 |  |  | 39.11 |  |  | 67,552 |  |  | 33.93 |
Forfeited | — |  |  | — |  |  | (1,230 | ) |  | 33.90 |
Nonvested units, March 31 | 131,159 |  |  | 36.46 |  |  | 66,322 |  |  | 33.93 |
Restricted Stock Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the restricted stock units activity was as follows:
| 2017 |  | 2016
Nonvested units, January 1 | 57,736 |  |  | — |
Granted | 54,688 |  |  | 57,904 |
Forfeited | — |  |  | (1,054 | )
Nonvested units, March 31 | 112,424 |  |  | 56,850 |
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three months ended March 31 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans 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 plans 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
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $1.2 |  |  |  | $1.3 |
Interest cost | 12.8 |  |  | 13.3 |  |  | 2.2 |  |  | 2.3 |
Expected return on plan assets | (16.4 | ) |  | (16.4 | ) |  | (1.5 | ) |  | (1.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (1.0 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 1.0 |  |  | 1.2 |
|  | $8.8 |  |  |  | $9.3 |  |  |  | $2.8 |  |  |  | $2.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.9 |  |  |  | $0.5 |  |  |  | $0.6 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 0.9 |  |  | 1.0 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service credit | — |  |  | — |  |  | — |  |  | (0.7 | )
Amortization of actuarial loss | 4.0 |  |  | 4.1 |  |  | 0.5 |  |  | 0.6 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $0.8 |  |  |  | $0.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.2 |  |  |  | $0.5 |  |  |  | $0.5 |
Interest cost | 5.5 |  |  | 5.6 |  |  | 0.9 |  |  | 0.9 |
Expected return on plan assets | (7.1 | ) |  | (7.1 | ) |  | (0.2 | ) |  | (0.2 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 0.4 |  |  | 0.5 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $1.5 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three months ended March 31, costs related to the 401(k) savings plan, 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 |  | 2017 |  | 2016 |  | 2017 |  | 2016
401(k) costs |  | $6.5 |  |  |  | $6.2 |  |  |  | $3.4 |  |  |  | $3.1 |  |  |  | $2.9 |  |  |  | $2.8 |
Recognized Compensation Expense And Income Tax Benefits | 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 months ended March 31 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $3.2 |  |  |  | $5.3 |  |  |  | $1.7 |  |  |  | $2.8 |  |  |  | $1.4 |  |  |  | $2.3 |
Income tax benefits | 1.3 |  |  | 2.2 |  |  | 0.7 |  |  | 1.1 |  |  | 0.5 |  |  | 0.9 |
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs for sponsored defined benefit pension and OPEB plans for the three months ended March 31 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans 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 plans 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
Alliant Energy | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $3.1 |  |  |  | $3.2 |  |  |  | $1.2 |  |  |  | $1.3 |
Interest cost | 12.8 |  |  | 13.3 |  |  | 2.2 |  |  | 2.3 |
Expected return on plan assets | (16.4 | ) |  | (16.4 | ) |  | (1.5 | ) |  | (1.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (1.0 | )
Amortization of actuarial loss | 9.4 |  |  | 9.3 |  |  | 1.0 |  |  | 1.2 |
|  | $8.8 |  |  |  | $9.3 |  |  |  | $2.8 |  |  |  | $2.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
IPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.8 |  |  |  | $1.9 |  |  |  | $0.5 |  |  |  | $0.6 |
Interest cost | 5.9 |  |  | 6.1 |  |  | 0.9 |  |  | 1.0 |
Expected return on plan assets | (7.7 | ) |  | (7.7 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service credit | — |  |  | — |  |  | — |  |  | (0.7 | )
Amortization of actuarial loss | 4.0 |  |  | 4.1 |  |  | 0.5 |  |  | 0.6 |
|  | $4.0 |  |  |  | $4.4 |  |  |  | $0.8 |  |  |  | $0.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
WPL | 2017 |  | 2016 |  | 2017 |  | 2016
Service cost |  | $1.2 |  |  |  | $1.2 |  |  |  | $0.5 |  |  |  | $0.5 |
Interest cost | 5.5 |  |  | 5.6 |  |  | 0.9 |  |  | 0.9 |
Expected return on plan assets | (7.1 | ) |  | (7.1 | ) |  | (0.2 | ) |  | (0.2 | )
Amortization of prior service cost (credit) | — |  |  | 0.1 |  |  | (0.1 | ) |  | (0.2 | )
Amortization of actuarial loss | 4.6 |  |  | 4.4 |  |  | 0.4 |  |  | 0.5 |
|  | $4.2 |  |  |  | $4.2 |  |  |  | $1.5 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three months ended March 31, costs related to the 401(k) savings plan, 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 |  | 2017 |  | 2016 |  | 2017 |  | 2016
401(k) costs |  | $6.5 |  |  |  | $6.2 |  |  |  | $3.4 |  |  |  | $3.1 |  |  |  | $2.9 |  |  |  | $2.8 |
Recognized Compensation Expense And Income Tax Benefits | 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 months ended March 31 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Compensation expense |  | $3.2 |  |  |  | $5.3 |  |  |  | $1.7 |  |  |  | $2.8 |  |  |  | $1.4 |  |  |  | $2.3 |
Income tax benefits | 1.3 |  |  | 2.2 |  |  | 0.7 |  |  | 1.1 |  |  | 0.5 |  |  | 0.9 |

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