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Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2015
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 nine months ended September 30 are included in the tables below (in millions). In the “IPL” and “WPL” tables below, the defined benefit pension plans costs represent those respective costs for IPL’s and WPL’s bargaining unit employees covered under the qualified plans that are sponsored by IPL and WPL, respectively, as well as amounts directly assigned to each of IPL and WPL 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 the “IPL” and “WPL” tables below, the OPEB plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively, as well as amounts directly assigned to each of IPL and WPL 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 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $4.0 |  |  |  | $3.3 |  |  |  | $11.9 |  |  |  | $9.9 |  |  |  | $1.3 |  |  |  | $1.3 |  |  |  | $4.1 |  |  |  | $3.9 |
Interest cost | 13.5 |  |  | 13.6 |  |  | 40.3 |  |  | 40.6 |  |  | 2.3 |  |  | 2.4 |  |  | 6.8 |  |  | 7.1 |
Expected return on plan assets | (18.7 | ) |  | (18.8 | ) |  | (56.2 | ) |  | (56.3 | ) |  | (2.1 | ) |  | (2.1 | ) |  | (6.3 | ) |  | (6.2 | )
Amortization of prior service credit | (0.1 | ) |  | — |  |  | (0.2 | ) |  | — |  |  | (2.8 | ) |  | (3.0 | ) |  | (8.4 | ) |  | (8.9 | )
Amortization of actuarial loss | 8.8 |  |  | 4.8 |  |  | 26.5 |  |  | 14.6 |  |  | 1.2 |  |  | 0.6 |  |  | 3.6 |  |  | 1.8 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $7.6 |  |  |  | $2.9 |  |  |  | $22.7 |  |  |  | $8.8 |  |  |  | ($0.1 | ) |  |  | ($0.8 | ) |  |  | ($0.2 | ) |  |  | ($2.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.6 |  |  |  | $5.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.8 |  |  |  | $1.8 |
Interest cost | 6.2 |  |  | 6.2 |  |  | 18.7 |  |  | 18.8 |  |  | 0.9 |  |  | 1.0 |  |  | 2.8 |  |  | 3.0 |
Expected return on plan assets | (8.9 | ) |  | (8.9 | ) |  | (26.8 | ) |  | (26.8 | ) |  | (1.4 | ) |  | (1.5 | ) |  | (4.2 | ) |  | (4.4 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | — |  |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 2.0 |  |  | 11.5 |  |  | 6.0 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.8 |
|  | $3.3 |  |  |  | $1.1 |  |  |  | $9.9 |  |  |  | $3.4 |  |  |  | ($0.8 | ) |  |  | ($1.2 | ) |  |  | ($2.5 | ) |  |  | ($3.5 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $1.4 |  |  |  | $1.3 |  |  |  | $4.3 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.5 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 16.9 |  |  | 17.0 |  |  | 0.9 |  |  | 1.0 |  |  | 2.7 |  |  | 2.9 |
Expected return on plan assets | (8.1 | ) |  | (8.1 | ) |  | (24.3 | ) |  | (24.3 | ) |  | (0.3 | ) |  | (0.4 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.2 |  |  | 0.2 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.6 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 2.3 |  |  | 12.6 |  |  | 6.9 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.9 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $3.3 |  |  |  | $1.2 |  |  |  | $10.1 |  |  |  | $3.5 |  |  |  | $0.8 |  |  |  | $0.4 |  |  |  | $2.3 |  |  |  | $1.4 |
Employees Participate In Defined Contribution Retirement Plans | For the three and nine months ended September 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 |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
401(k) costs |  | $6.4 |  |  |  | $5.3 |  |  |  | $18.7 |  |  |  | $17.3 |  |  |  | $3.3 |  |  |  | $2.7 |  |  |  | $9.6 |  |  |  | $8.4 |  |  |  | $2.9 |  |  |  | $2.4 |  |  |  | $8.4 |  |  |  | $8.3 |
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 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
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Compensation expense |  | $0.3 |  |  |  | $1.2 |  |  |  | $5.8 |  |  |  | $7.4 |  |  |  | $0.2 |  |  |  | $0.6 |  |  |  | $3.1 |  |  |  | $4.0 |  |  |  | $0.1 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $3.1 |
Income tax benefits | 0.2 |  |  | 0.5 |  |  | 2.4 |  |  | 3.0 |  |  | 0.1 |  |  | 0.2 |  |  | 1.3 |  |  | 1.6 |  |  | — |  |  | 0.3 |  |  | 1.0 |  |  | 1.3 |
Schedule Of Equity-based Compensation Plans Activity | Information related to fair values of nonvested performance shares and units at September 30, 2015, by year of grant, was as follows:
| Performance Shares |  | Performance Units
| 2015 Grant |  | 2014 Grant |  | 2013 Grant |  | 2015 Grant |  | 2014 Grant |  | 2013 Grant
Nonvested awards | 45,403 |  |  | 49,719 |  |  | 49,093 |  |  | 17,386 |  |  | 19,440 |  |  | 21,380 |
Alliant Energy common stock closing price on September 30, 2015 |  | $58.49 |  |  |  | $58.49 |  |  |  | $58.49 |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date |  |  |  |  |  |  |  | $65.09 |  |  |  | $53.77 |  |  |  | $47.58 |
Estimated payout percentage based on performance criteria | 70 | % |  | 105 | % |  | 150 | % |  | 70 | % |  | 105 | % |  | 150 | %
Fair values of each nonvested award |  | $40.94 |  |  |  | $61.41 |  |  |  | $87.74 |  |  |  | $45.56 |  |  |  | $56.46 |  |  |  | $71.37 |
Performance Shares [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance shares activity, with share amounts representing the target number of performance shares, was as follows:
| 2015 |  | 2014
Nonvested shares, January 1 | 144,424 |  |  | 139,940 |
Granted | 45,403 |  |  | 51,221 |
Vested | (45,612 | ) |  | (45,235 | )
Forfeited | — |  |  | (1,502 | )
Nonvested shares, September 30 | 144,215 |  |  | 144,424 |
During the nine months ended September 30, certain performance shares vested, resulting in payouts (a combination of cash and common stock) as follows:
| 2015 |  | 2014
| 2012 Grant |  | 2011 Grant
Performance shares vested | 45,612 |  |  | 45,235 |
Percentage of target number of performance shares | 167.5 | % |  | 147.5 | %
Aggregate payout value (in millions) |  | $5.1 |  |  |  | $3.4 |
Payout - cash (in millions) |  | $3.2 |  |  |  | $2.9 |
Payout - common stock shares issued | 10,975 |  |  | 4,810 |
Performance Units [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity-based Compensation Plans Activity | A summary of the performance units activity, with unit amounts representing the target number of performance units, was as follows:
| 2015 |  | 2014
Nonvested units, January 1 | 63,665 |  |  | 65,912 |
Granted | 17,837 |  |  | 20,422 |
Vested | (22,845 | ) |  | (20,751 | )
Forfeited | (451 | ) |  | (958 | )
Nonvested units, September 30 | 58,206 |  |  | 64,625 |
During the nine months ended September 30, certain performance units vested, resulting in cash payouts as follows:
| 2015 |  | 2014
| 2012 Grant |  | 2011 Grant
Performance units vested | 22,845 |  |  | 20,751 |
Percentage of target number of performance units | 167.5 | % |  | 147.5 | %
Payout value (in millions) |  | $1.6 |  |  |  | $1.2 |
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:
| 2015 |  | 2014
| Shares |  | Weighted Average Fair Value |  | Shares |  | Weighted Average Fair Value
Nonvested shares, January 1 | 98,812 |  |  |  | $50.69 |  |  | 158,922 |  |  |  | $42.71 |
Granted | 45,403 |  |  | 65.09 |  |  | 51,221 |  |  | 53.77 |
Vested (a) | (49,093 | ) |  | 47.58 |  |  | (90,847 | ) |  | 40.91 |
Forfeited | — |  |  | — |  |  | (20,484 | ) |  | 39.85 |
Nonvested shares, September 30 | 95,122 |  |  | 59.17 |  |  | 98,812 |  |  | 50.69 |
(a) | In 2015, 49,093 performance contingent restricted shares granted in 2013 vested because the specified performance criteria for such shares were met. In 2014, 45,612 and 45,235 performance contingent restricted shares granted in 2012 and 2011, respectively, vested because the specified performance criteria for such shares were met.
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:
| 2015 |  | 2014
Nonvested awards, January 1 | 78,930 |  |  | 96,977 |
Granted | 41,105 |  |  | 42,446 |
Vested (a) | (37,332 | ) |  | (55,517 | )
Forfeited | (827 | ) |  | (4,295 | )
Nonvested awards, September 30 | 81,876 |  |  | 79,611 |
(a) | In 2015, 37,332 performance contingent cash awards granted in 2013 vested, resulting in cash payouts valued at $2.4 million. In 2014, 34,766 and 20,751 performance contingent cash awards granted in 2012 and 2011 vested, resulting in cash payouts valued at $1.9 million and $1.1 million, respectively.
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 nine months ended September 30 are included in the tables below (in millions). In the “IPL” and “WPL” tables below, the defined benefit pension plans costs represent those respective costs for IPL’s and WPL’s bargaining unit employees covered under the qualified plans that are sponsored by IPL and WPL, respectively, as well as amounts directly assigned to each of IPL and WPL 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 the “IPL” and “WPL” tables below, the OPEB plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively, as well as amounts directly assigned to each of IPL and WPL 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 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $4.0 |  |  |  | $3.3 |  |  |  | $11.9 |  |  |  | $9.9 |  |  |  | $1.3 |  |  |  | $1.3 |  |  |  | $4.1 |  |  |  | $3.9 |
Interest cost | 13.5 |  |  | 13.6 |  |  | 40.3 |  |  | 40.6 |  |  | 2.3 |  |  | 2.4 |  |  | 6.8 |  |  | 7.1 |
Expected return on plan assets | (18.7 | ) |  | (18.8 | ) |  | (56.2 | ) |  | (56.3 | ) |  | (2.1 | ) |  | (2.1 | ) |  | (6.3 | ) |  | (6.2 | )
Amortization of prior service credit | (0.1 | ) |  | — |  |  | (0.2 | ) |  | — |  |  | (2.8 | ) |  | (3.0 | ) |  | (8.4 | ) |  | (8.9 | )
Amortization of actuarial loss | 8.8 |  |  | 4.8 |  |  | 26.5 |  |  | 14.6 |  |  | 1.2 |  |  | 0.6 |  |  | 3.6 |  |  | 1.8 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $7.6 |  |  |  | $2.9 |  |  |  | $22.7 |  |  |  | $8.8 |  |  |  | ($0.1 | ) |  |  | ($0.8 | ) |  |  | ($0.2 | ) |  |  | ($2.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.6 |  |  |  | $5.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.8 |  |  |  | $1.8 |
Interest cost | 6.2 |  |  | 6.2 |  |  | 18.7 |  |  | 18.8 |  |  | 0.9 |  |  | 1.0 |  |  | 2.8 |  |  | 3.0 |
Expected return on plan assets | (8.9 | ) |  | (8.9 | ) |  | (26.8 | ) |  | (26.8 | ) |  | (1.4 | ) |  | (1.5 | ) |  | (4.2 | ) |  | (4.4 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | — |  |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 2.0 |  |  | 11.5 |  |  | 6.0 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.8 |
|  | $3.3 |  |  |  | $1.1 |  |  |  | $9.9 |  |  |  | $3.4 |  |  |  | ($0.8 | ) |  |  | ($1.2 | ) |  |  | ($2.5 | ) |  |  | ($3.5 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $1.4 |  |  |  | $1.3 |  |  |  | $4.3 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.5 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 16.9 |  |  | 17.0 |  |  | 0.9 |  |  | 1.0 |  |  | 2.7 |  |  | 2.9 |
Expected return on plan assets | (8.1 | ) |  | (8.1 | ) |  | (24.3 | ) |  | (24.3 | ) |  | (0.3 | ) |  | (0.4 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.2 |  |  | 0.2 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.6 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 2.3 |  |  | 12.6 |  |  | 6.9 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.9 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $3.3 |  |  |  | $1.2 |  |  |  | $10.1 |  |  |  | $3.5 |  |  |  | $0.8 |  |  |  | $0.4 |  |  |  | $2.3 |  |  |  | $1.4 |
Employees Participate In Defined Contribution Retirement Plans | For the three and nine months ended September 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 |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
401(k) costs |  | $6.4 |  |  |  | $5.3 |  |  |  | $18.7 |  |  |  | $17.3 |  |  |  | $3.3 |  |  |  | $2.7 |  |  |  | $9.6 |  |  |  | $8.4 |  |  |  | $2.9 |  |  |  | $2.4 |  |  |  | $8.4 |  |  |  | $8.3 |
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 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
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Compensation expense |  | $0.3 |  |  |  | $1.2 |  |  |  | $5.8 |  |  |  | $7.4 |  |  |  | $0.2 |  |  |  | $0.6 |  |  |  | $3.1 |  |  |  | $4.0 |  |  |  | $0.1 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $3.1 |
Income tax benefits | 0.2 |  |  | 0.5 |  |  | 2.4 |  |  | 3.0 |  |  | 0.1 |  |  | 0.2 |  |  | 1.3 |  |  | 1.6 |  |  | — |  |  | 0.3 |  |  | 1.0 |  |  | 1.3 |
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 nine months ended September 30 are included in the tables below (in millions). In the “IPL” and “WPL” tables below, the defined benefit pension plans costs represent those respective costs for IPL’s and WPL’s bargaining unit employees covered under the qualified plans that are sponsored by IPL and WPL, respectively, as well as amounts directly assigned to each of IPL and WPL 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 the “IPL” and “WPL” tables below, the OPEB plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively, as well as amounts directly assigned to each of IPL and WPL 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 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $4.0 |  |  |  | $3.3 |  |  |  | $11.9 |  |  |  | $9.9 |  |  |  | $1.3 |  |  |  | $1.3 |  |  |  | $4.1 |  |  |  | $3.9 |
Interest cost | 13.5 |  |  | 13.6 |  |  | 40.3 |  |  | 40.6 |  |  | 2.3 |  |  | 2.4 |  |  | 6.8 |  |  | 7.1 |
Expected return on plan assets | (18.7 | ) |  | (18.8 | ) |  | (56.2 | ) |  | (56.3 | ) |  | (2.1 | ) |  | (2.1 | ) |  | (6.3 | ) |  | (6.2 | )
Amortization of prior service credit | (0.1 | ) |  | — |  |  | (0.2 | ) |  | — |  |  | (2.8 | ) |  | (3.0 | ) |  | (8.4 | ) |  | (8.9 | )
Amortization of actuarial loss | 8.8 |  |  | 4.8 |  |  | 26.5 |  |  | 14.6 |  |  | 1.2 |  |  | 0.6 |  |  | 3.6 |  |  | 1.8 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $7.6 |  |  |  | $2.9 |  |  |  | $22.7 |  |  |  | $8.8 |  |  |  | ($0.1 | ) |  |  | ($0.8 | ) |  |  | ($0.2 | ) |  |  | ($2.3 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
IPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.6 |  |  |  | $5.4 |  |  |  | $0.6 |  |  |  | $0.6 |  |  |  | $1.8 |  |  |  | $1.8 |
Interest cost | 6.2 |  |  | 6.2 |  |  | 18.7 |  |  | 18.8 |  |  | 0.9 |  |  | 1.0 |  |  | 2.8 |  |  | 3.0 |
Expected return on plan assets | (8.9 | ) |  | (8.9 | ) |  | (26.8 | ) |  | (26.8 | ) |  | (1.4 | ) |  | (1.5 | ) |  | (4.2 | ) |  | (4.4 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | — |  |  | (1.5 | ) |  | (1.6 | ) |  | (4.6 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 2.0 |  |  | 11.5 |  |  | 6.0 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.8 |
|  | $3.3 |  |  |  | $1.1 |  |  |  | $9.9 |  |  |  | $3.4 |  |  |  | ($0.8 | ) |  |  | ($1.2 | ) |  |  | ($2.5 | ) |  |  | ($3.5 | )
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
WPL | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Service cost |  | $1.4 |  |  |  | $1.3 |  |  |  | $4.3 |  |  |  | $3.7 |  |  |  | $0.5 |  |  |  | $0.5 |  |  |  | $1.6 |  |  |  | $1.5 |
Interest cost | 5.6 |  |  | 5.7 |  |  | 16.9 |  |  | 17.0 |  |  | 0.9 |  |  | 1.0 |  |  | 2.7 |  |  | 2.9 |
Expected return on plan assets | (8.1 | ) |  | (8.1 | ) |  | (24.3 | ) |  | (24.3 | ) |  | (0.3 | ) |  | (0.4 | ) |  | (1.1 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | — |  |  | 0.2 |  |  | 0.2 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.6 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 2.3 |  |  | 12.6 |  |  | 6.9 |  |  | 0.6 |  |  | 0.3 |  |  | 1.7 |  |  | 0.9 |
Additional benefit costs | 0.1 |  |  | — |  |  | 0.4 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $3.3 |  |  |  | $1.2 |  |  |  | $10.1 |  |  |  | $3.5 |  |  |  | $0.8 |  |  |  | $0.4 |  |  |  | $2.3 |  |  |  | $1.4 |
Employees Participate In Defined Contribution Retirement Plans | For the three and nine months ended September 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 |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
401(k) costs |  | $6.4 |  |  |  | $5.3 |  |  |  | $18.7 |  |  |  | $17.3 |  |  |  | $3.3 |  |  |  | $2.7 |  |  |  | $9.6 |  |  |  | $8.4 |  |  |  | $2.9 |  |  |  | $2.4 |  |  |  | $8.4 |  |  |  | $8.3 |
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 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
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Compensation expense |  | $0.3 |  |  |  | $1.2 |  |  |  | $5.8 |  |  |  | $7.4 |  |  |  | $0.2 |  |  |  | $0.6 |  |  |  | $3.1 |  |  |  | $4.0 |  |  |  | $0.1 |  |  |  | $0.5 |  |  |  | $2.5 |  |  |  | $3.1 |
Income tax benefits | 0.2 |  |  | 0.5 |  |  | 2.4 |  |  | 3.0 |  |  | 0.1 |  |  | 0.2 |  |  | 1.3 |  |  | 1.6 |  |  | — |  |  | 0.3 |  |  | 1.0 |  |  | 1.3 |

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