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Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans for the three and six months ended June 30 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans costs represent those respective costs 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 costs (credits) represent respective costs (credits) 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 |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $3.1 |  |  |  | $3.9 |  |  |  | $6.3 |  |  |  | $7.9 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.6 |  |  |  | $2.8 |
Interest cost | 13.2 |  |  | 13.4 |  |  | 26.5 |  |  | 26.8 |  |  | 2.4 |  |  | 2.3 |  |  | 4.7 |  |  | 4.5 |
Expected return on plan assets | (16.4 | ) |  | (18.8 | ) |  | (32.8 | ) |  | (37.5 | ) |  | (1.5 | ) |  | (2.1 | ) |  | (3.0 | ) |  | (4.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (1.1 | ) |  | (2.8 | ) |  | (2.1 | ) |  | (5.6 | )
Amortization of actuarial loss | 9.4 |  |  | 8.9 |  |  | 18.7 |  |  | 17.7 |  |  | 1.2 |  |  | 1.2 |  |  | 2.4 |  |  | 2.4 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.3 |  |  |  | $7.5 |  |  |  | $18.6 |  |  |  | $15.1 |  |  |  | $2.3 |  |  |  | $— |  |  |  | $4.6 |  |  |  | ($0.1 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.9 |  |  |  | $2.2 |  |  |  | $3.8 |  |  |  | $4.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.2 |  |  |  | $1.2 |
Interest cost | 6.2 |  |  | 6.3 |  |  | 12.3 |  |  | 12.5 |  |  | 0.9 |  |  | 1.0 |  |  | 1.9 |  |  | 1.9 |
Expected return on plan assets | (7.8 | ) |  | (9.0 | ) |  | (15.5 | ) |  | (17.9 | ) |  | (1.2 | ) |  | (1.4 | ) |  | (2.2 | ) |  | (2.8 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.6 | ) |  | (1.6 | ) |  | (1.3 | ) |  | (3.1 | )
Amortization of actuarial loss | 4.1 |  |  | 3.9 |  |  | 8.2 |  |  | 7.7 |  |  | 0.7 |  |  | 0.5 |  |  | 1.3 |  |  | 1.1 |
|  | $4.3 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $6.6 |  |  |  | $0.4 |  |  |  | ($0.9 | ) |  |  | $0.9 |  |  |  | ($1.7 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.2 |  |  |  | $1.5 |  |  |  | $2.4 |  |  |  | $2.9 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.0 |  |  |  | $1.1 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 11.2 |  |  | 11.3 |  |  | 1.0 |  |  | 0.9 |  |  | 1.9 |  |  | 1.8 |
Expected return on plan assets | (7.1 | ) |  | (8.1 | ) |  | (14.2 | ) |  | (16.2 | ) |  | (0.2 | ) |  | (0.4 | ) |  | (0.4 | ) |  | (0.8 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.2 | ) |  | (0.8 | ) |  | (0.4 | ) |  | (1.7 | )
Amortization of actuarial loss | 4.4 |  |  | 4.2 |  |  | 8.8 |  |  | 8.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.1 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.1 |  |  |  | $3.4 |  |  |  | $8.3 |  |  |  | $6.8 |  |  |  | $1.5 |  |  |  | $0.7 |  |  |  | $3.0 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three and six months ended June 30, 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
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
401(k) costs |  | $5.7 |  |  |  | $5.6 |  |  |  | $11.9 |  |  |  | $12.3 |  |  |  | $2.9 |  |  |  | $2.9 |  |  |  | $6.0 |  |  |  | $6.3 |  |  |  | $2.6 |  |  |  | $2.6 |  |  |  | $5.4 |  |  |  | $5.5 |
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 and six months ended June 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Compensation expense |  | $7.1 |  |  |  | $2.3 |  |  |  | $12.4 |  |  |  | $5.5 |  |  |  | $3.7 |  |  |  | $1.2 |  |  |  | $6.5 |  |  |  | $2.9 |  |  |  | $3.1 |  |  |  | $1.0 |  |  |  | $5.4 |  |  |  | $2.4 |
Income tax benefits | 2.9 |  |  | 0.9 |  |  | 5.1 |  |  | 2.2 |  |  | 1.6 |  |  | 0.5 |  |  | 2.7 |  |  | 1.2 |  |  | 1.3 |  |  | 0.4 |  |  | 2.2 |  |  | 1.0 |
Performance Shares and 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
| 2016 |  | 2015 |  | 2016 |  | 2015
Nonvested awards, January 1 | 288,430 |  |  | 288,848 |  |  | 116,412 |  |  | 127,330 |
Granted | 68,585 |  |  | 90,806 |  |  | 23,918 |  |  | 35,674 |
Vested | (98,186 | ) |  | (91,224 | ) |  | (42,760 | ) |  | (45,690 | )
Forfeited | (1,230 | ) |  | — |  |  | (4,139 | ) |  | (666 | )
Nonvested awards, June 30 | 257,599 |  |  | 288,430 |  |  | 93,431 |  |  | 116,648 |
Granted Awards - For performance units granted in 2016, the final value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. 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 - During the six months ended June 30, 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
| 2016 |  | 2015 |  | 2016 |  | 2015
| 2013 Grant |  | 2012 Grant |  | 2013 Grant |  | 2012 Grant
Performance awards vested | 98,186 |  |  | 91,224 |  |  | 42,760 |  |  | 45,690 |
Percentage of target number of performance awards | 165.0 | % |  | 167.5 | % |  | 165.0 | % |  | 167.5 | %
Aggregate payout value (in millions) |  | $5.1 |  |  |  | $5.1 |  |  |  | $1.7 |  |  |  | $1.6 |
Payout - cash (in millions) |  | $2.9 |  |  |  | $3.2 |  |  |  | $1.7 |  |  |  | $1.6 |
Payout - common stock shares issued | 22,408 |  |  | 21,950 |  |  | N/A |  | N/A
Fair Value of Awards - Information related to fair values of nonvested performance shares and performance units at June 30, 2016, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2016 Grant |  | 2015 Grant |  | 2014 Grant |  | 2016 Grant |  | 2015 Grant |  | 2014 Grant
Nonvested awards | 67,355 |  |  | 90,806 |  |  | 99,438 |  |  | 22,768 |  |  | 33,268 |  |  | 37,395 |
Alliant Energy common stock closing price on June 30, 2016 |  | $39.70 |  |  |  | $39.70 |  |  |  | $39.70 |  |  |  | $39.70 |  |  | N/A |  | N/A
Alliant Energy common stock closing price on grant date | N/A |  | N/A |  | N/A |  | N/A |  |  | $32.55 |  |  |  | $26.89 |
Estimated payout percentage based on performance criteria | 118 | % |  | 150 | % |  | 155 | % |  | 118 | % |  | 150 | % |  | 155 | %
Fair values of each nonvested award |  | $46.85 |  |  |  | $59.55 |  |  |  | $61.54 |  |  |  | $46.85 |  |  |  | $48.82 |  |  |  | $41.67 |
Performance Contingent Restricted Stock [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance contingent restricted stock activity was as follows:
| 2016 |  | 2015
| Shares |  | Weighted Average Grant Date Fair Value |  | Shares |  | Weighted Average Grant Date Fair Value
Nonvested shares, January 1 | 190,244 |  |  |  | $29.59 |  |  | 197,624 |  |  |  | $25.35 |
Granted | — |  |  | — |  |  | 90,806 |  |  | 32.55 |
Vested (a) | — |  |  | — |  |  | (98,186 | ) |  | 23.79 |
Nonvested shares, June 30 | 190,244 |  |  | 29.59 |  |  | 190,244 |  |  | 29.59 |
(a) | In 2015, 98,186 performance contingent restricted shares granted in 2013 vested because the specified performance criteria for such shares were met.
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:
| 2016
| Units |  | Weighted Average Grant Date Fair Value
Granted | 68,585 |  |  |  | $33.96 |
Forfeited | (1,230 | ) |  | 33.90 |
Nonvested units, June 30 | 67,355 |  |  | 33.96 |
Performance Restricted Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance restricted units activity, with amounts representing the target number of units, was as follows:
| 2016
Granted | 23,918 |
Forfeited | (1,150 | )
Nonvested units, June 30 | 22,768 |
Key Employee Performance Restricted Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the key employee performance restricted units activity, with amounts representing the target number of units, was as follows:
| 2016
Granted | 45,056 |
Forfeited | (1,238 | )
Nonvested units, June 30 | 43,818 |
Restricted Stock Unit And Restricted Unit [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the restricted stock units and restricted units activity was as follows:
| 2016
| Restricted Stock Units |  | Restricted Units
Granted | 58,790 |  |  | 20,502 |
Forfeited | (1,054 | ) |  | (986 | )
Nonvested units, June 30 | 57,736 |  |  | 19,516 |
Performance Contingent Cash Awards [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance-contingent cash awards activity was as follows:
| 2016 |  | 2015
Nonvested awards, January 1 | 163,752 |  |  | 157,860 |
Granted | — |  |  | 82,210 |
Vested (a) | — |  |  | (74,664 | )
Forfeited | (3,652 | ) |  | (1,418 | )
Nonvested awards, June 30 | 160,100 |  |  | 163,988 |
(a) | In 2015, 74,664 performance-contingent cash awards granted in 2013 vested, resulting in cash payouts valued at $2.4 million.
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans for the three and six months ended June 30 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans costs represent those respective costs 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 costs (credits) represent respective costs (credits) 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 |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $3.1 |  |  |  | $3.9 |  |  |  | $6.3 |  |  |  | $7.9 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.6 |  |  |  | $2.8 |
Interest cost | 13.2 |  |  | 13.4 |  |  | 26.5 |  |  | 26.8 |  |  | 2.4 |  |  | 2.3 |  |  | 4.7 |  |  | 4.5 |
Expected return on plan assets | (16.4 | ) |  | (18.8 | ) |  | (32.8 | ) |  | (37.5 | ) |  | (1.5 | ) |  | (2.1 | ) |  | (3.0 | ) |  | (4.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (1.1 | ) |  | (2.8 | ) |  | (2.1 | ) |  | (5.6 | )
Amortization of actuarial loss | 9.4 |  |  | 8.9 |  |  | 18.7 |  |  | 17.7 |  |  | 1.2 |  |  | 1.2 |  |  | 2.4 |  |  | 2.4 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.3 |  |  |  | $7.5 |  |  |  | $18.6 |  |  |  | $15.1 |  |  |  | $2.3 |  |  |  | $— |  |  |  | $4.6 |  |  |  | ($0.1 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.9 |  |  |  | $2.2 |  |  |  | $3.8 |  |  |  | $4.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.2 |  |  |  | $1.2 |
Interest cost | 6.2 |  |  | 6.3 |  |  | 12.3 |  |  | 12.5 |  |  | 0.9 |  |  | 1.0 |  |  | 1.9 |  |  | 1.9 |
Expected return on plan assets | (7.8 | ) |  | (9.0 | ) |  | (15.5 | ) |  | (17.9 | ) |  | (1.2 | ) |  | (1.4 | ) |  | (2.2 | ) |  | (2.8 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.6 | ) |  | (1.6 | ) |  | (1.3 | ) |  | (3.1 | )
Amortization of actuarial loss | 4.1 |  |  | 3.9 |  |  | 8.2 |  |  | 7.7 |  |  | 0.7 |  |  | 0.5 |  |  | 1.3 |  |  | 1.1 |
|  | $4.3 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $6.6 |  |  |  | $0.4 |  |  |  | ($0.9 | ) |  |  | $0.9 |  |  |  | ($1.7 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.2 |  |  |  | $1.5 |  |  |  | $2.4 |  |  |  | $2.9 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.0 |  |  |  | $1.1 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 11.2 |  |  | 11.3 |  |  | 1.0 |  |  | 0.9 |  |  | 1.9 |  |  | 1.8 |
Expected return on plan assets | (7.1 | ) |  | (8.1 | ) |  | (14.2 | ) |  | (16.2 | ) |  | (0.2 | ) |  | (0.4 | ) |  | (0.4 | ) |  | (0.8 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.2 | ) |  | (0.8 | ) |  | (0.4 | ) |  | (1.7 | )
Amortization of actuarial loss | 4.4 |  |  | 4.2 |  |  | 8.8 |  |  | 8.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.1 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.1 |  |  |  | $3.4 |  |  |  | $8.3 |  |  |  | $6.8 |  |  |  | $1.5 |  |  |  | $0.7 |  |  |  | $3.0 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three and six months ended June 30, 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
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
401(k) costs |  | $5.7 |  |  |  | $5.6 |  |  |  | $11.9 |  |  |  | $12.3 |  |  |  | $2.9 |  |  |  | $2.9 |  |  |  | $6.0 |  |  |  | $6.3 |  |  |  | $2.6 |  |  |  | $2.6 |  |  |  | $5.4 |  |  |  | $5.5 |
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 and six months ended June 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Compensation expense |  | $7.1 |  |  |  | $2.3 |  |  |  | $12.4 |  |  |  | $5.5 |  |  |  | $3.7 |  |  |  | $1.2 |  |  |  | $6.5 |  |  |  | $2.9 |  |  |  | $3.1 |  |  |  | $1.0 |  |  |  | $5.4 |  |  |  | $2.4 |
Income tax benefits | 2.9 |  |  | 0.9 |  |  | 5.1 |  |  | 2.2 |  |  | 1.6 |  |  | 0.5 |  |  | 2.7 |  |  | 1.2 |  |  | 1.3 |  |  | 0.4 |  |  | 2.2 |  |  | 1.0 |
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs (credits) for sponsored defined benefit pension and OPEB plans for the three and six months ended June 30 are included in the tables below (in millions). In IPL’s and WPL’s tables below, the defined benefit pension plans costs represent those respective costs 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 costs (credits) represent respective costs (credits) 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 |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $3.1 |  |  |  | $3.9 |  |  |  | $6.3 |  |  |  | $7.9 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.6 |  |  |  | $2.8 |
Interest cost | 13.2 |  |  | 13.4 |  |  | 26.5 |  |  | 26.8 |  |  | 2.4 |  |  | 2.3 |  |  | 4.7 |  |  | 4.5 |
Expected return on plan assets | (16.4 | ) |  | (18.8 | ) |  | (32.8 | ) |  | (37.5 | ) |  | (1.5 | ) |  | (2.1 | ) |  | (3.0 | ) |  | (4.2 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (1.1 | ) |  | (2.8 | ) |  | (2.1 | ) |  | (5.6 | )
Amortization of actuarial loss | 9.4 |  |  | 8.9 |  |  | 18.7 |  |  | 17.7 |  |  | 1.2 |  |  | 1.2 |  |  | 2.4 |  |  | 2.4 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $9.3 |  |  |  | $7.5 |  |  |  | $18.6 |  |  |  | $15.1 |  |  |  | $2.3 |  |  |  | $— |  |  |  | $4.6 |  |  |  | ($0.1 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.9 |  |  |  | $2.2 |  |  |  | $3.8 |  |  |  | $4.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.2 |  |  |  | $1.2 |
Interest cost | 6.2 |  |  | 6.3 |  |  | 12.3 |  |  | 12.5 |  |  | 0.9 |  |  | 1.0 |  |  | 1.9 |  |  | 1.9 |
Expected return on plan assets | (7.8 | ) |  | (9.0 | ) |  | (15.5 | ) |  | (17.9 | ) |  | (1.2 | ) |  | (1.4 | ) |  | (2.2 | ) |  | (2.8 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.6 | ) |  | (1.6 | ) |  | (1.3 | ) |  | (3.1 | )
Amortization of actuarial loss | 4.1 |  |  | 3.9 |  |  | 8.2 |  |  | 7.7 |  |  | 0.7 |  |  | 0.5 |  |  | 1.3 |  |  | 1.1 |
|  | $4.3 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $6.6 |  |  |  | $0.4 |  |  |  | ($0.9 | ) |  |  | $0.9 |  |  |  | ($1.7 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Service cost |  | $1.2 |  |  |  | $1.5 |  |  |  | $2.4 |  |  |  | $2.9 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.0 |  |  |  | $1.1 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 11.2 |  |  | 11.3 |  |  | 1.0 |  |  | 0.9 |  |  | 1.9 |  |  | 1.8 |
Expected return on plan assets | (7.1 | ) |  | (8.1 | ) |  | (14.2 | ) |  | (16.2 | ) |  | (0.2 | ) |  | (0.4 | ) |  | (0.4 | ) |  | (0.8 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (0.2 | ) |  | (0.8 | ) |  | (0.4 | ) |  | (1.7 | )
Amortization of actuarial loss | 4.4 |  |  | 4.2 |  |  | 8.8 |  |  | 8.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.1 |
Additional benefit costs | — |  |  | 0.1 |  |  | — |  |  | 0.3 |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.1 |  |  |  | $3.4 |  |  |  | $8.3 |  |  |  | $6.8 |  |  |  | $1.5 |  |  |  | $0.7 |  |  |  | $3.0 |  |  |  | $1.5 |
Employees Participate In Defined Contribution Retirement Plans | For the three and six months ended June 30, 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
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
401(k) costs |  | $5.7 |  |  |  | $5.6 |  |  |  | $11.9 |  |  |  | $12.3 |  |  |  | $2.9 |  |  |  | $2.9 |  |  |  | $6.0 |  |  |  | $6.3 |  |  |  | $2.6 |  |  |  | $2.6 |  |  |  | $5.4 |  |  |  | $5.5 |
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 and six months ended June 30 was as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Three Months |  | Six Months |  | Three Months |  | Six Months |  | Three Months |  | Six Months
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Compensation expense |  | $7.1 |  |  |  | $2.3 |  |  |  | $12.4 |  |  |  | $5.5 |  |  |  | $3.7 |  |  |  | $1.2 |  |  |  | $6.5 |  |  |  | $2.9 |  |  |  | $3.1 |  |  |  | $1.0 |  |  |  | $5.4 |  |  |  | $2.4 |
Income tax benefits | 2.9 |  |  | 0.9 |  |  | 5.1 |  |  | 2.2 |  |  | 1.6 |  |  | 0.5 |  |  | 2.7 |  |  | 1.2 |  |  | 1.3 |  |  | 0.4 |  |  | 2.2 |  |  | 1.0 |

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