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Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2013
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | The components of net periodic benefit costs (credits) for Alliant Energy’s, IPL’s and WPL’s sponsored defined benefit pension and other postretirement benefits 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 IPL’s and WPL’s 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 other postretirement benefits plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively.
Alliant Energy | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $3.9 |  |  |  | $3.3 |  |  |  | $11.8 |  |  |  | $10.0 |  |  |  | $1.5 |  |  |  | $1.7 |  |  |  | $4.7 |  |  |  | $5.2 |
Interest cost | 12.3 |  |  | 13.0 |  |  | 36.8 |  |  | 38.9 |  |  | 2.1 |  |  | 2.6 |  |  | 6.3 |  |  | 7.7 |
Expected return on plan assets | (18.5 | ) |  | (17.2 | ) |  | (55.5 | ) |  | (51.6 | ) |  | (2.0 | ) |  | (1.9 | ) |  | (6.0 | ) |  | (5.7 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (3.0 | ) |  | (3.0 | ) |  | (8.9 | ) |  | (9.0 | )
Amortization of actuarial loss | 9.0 |  |  | 8.3 |  |  | 27.1 |  |  | 24.9 |  |  | 1.3 |  |  | 1.6 |  |  | 3.7 |  |  | 4.7 |
|  | $6.8 |  |  |  | $7.5 |  |  |  | $20.4 |  |  |  | $22.4 |  |  |  | ($0.1 | ) |  |  | $1.0 |  |  |  | ($0.2 | ) |  |  | $2.9 |
IPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.5 |  |  |  | $5.6 |  |  |  | $0.8 |  |  |  | $0.7 |  |  |  | $2.2 |  |  |  | $2.2 |
Interest cost | 5.7 |  |  | 6.1 |  |  | 17.1 |  |  | 18.1 |  |  | 0.9 |  |  | 1.1 |  |  | 2.7 |  |  | 3.3 |
Expected return on plan assets | (8.8 | ) |  | (8.2 | ) |  | (26.4 | ) |  | (24.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (4.2 | ) |  | (3.9 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (1.6 | ) |  | (1.5 | ) |  | (4.7 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 3.6 |  |  | 11.4 |  |  | 10.6 |  |  | 0.6 |  |  | 0.9 |  |  | 2.0 |  |  | 2.7 |
|  | $2.9 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $9.9 |  |  |  | ($0.7 | ) |  |  | ($0.1 | ) |  |  | ($2.0 | ) |  |  | ($0.4 | )
WPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $1.5 |  |  |  | $1.3 |  |  |  | $4.4 |  |  |  | $3.9 |  |  |  | $0.6 |  |  |  | $0.7 |  |  |  | $1.8 |  |  |  | $2.1 |
Interest cost | 5.2 |  |  | 5.4 |  |  | 15.5 |  |  | 16.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 3.1 |
Expected return on plan assets | (8.0 | ) |  | (7.5 | ) |  | (23.9 | ) |  | (22.3 | ) |  | (0.4 | ) |  | (0.3 | ) |  | (1.0 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.2 |  |  | 0.3 |  |  | 0.3 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.9 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 3.9 |  |  | 12.8 |  |  | 11.8 |  |  | 0.5 |  |  | 0.6 |  |  | 1.5 |  |  | 1.7 |
|  | $3.0 |  |  |  | $3.3 |  |  |  | $9.1 |  |  |  | $9.9 |  |  |  | $0.6 |  |  |  | $1.0 |  |  |  | $1.9 |  |  |  | $3.0 |
Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs | The following table includes the allocated qualified and non-qualified pension and other postretirement benefits costs (credits) associated with Corporate Services employees providing services to IPL and WPL for the three and nine months ended September 30 (in millions):
| Pension Benefits Costs |  | Other Postretirement Benefits Costs (Credits)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
IPL |  | $0.4 |  |  |  | $0.4 |  |  |  | $1.4 |  |  |  | $1.4 |  |  |  | ($0.1 | ) |  |  | $— |  |  |  | ($0.2 | ) |  |  | $0.1 |
WPL | 0.4 |  |  | 0.4 |  |  | 1.1 |  |  | 1.0 |  |  | (0.1 | ) |  | 0.1 |  |  | (0.2 | ) |  | 0.1 |
Estimated Future And Actual Employer Contributions | Estimated and actual funding for the qualified and non-qualified defined benefit pension and other postretirement benefits plans for 2013 are as follows (in millions):
| Estimated for Calendar Year 2013 |  | Actual Through September 30, 2013
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $2.3 |  |  |  | $0.8 |  |  |  | $0.2 |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $0.2 |
Other postretirement benefits plans | 3.0 |  |  | — |  |  | 3.0 |  |  | 2.0 |  |  | — |  |  | 2.0 |
(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.
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’ level of contribution, were as follows (in millions):
| Alliant Energy |  | IPL (a) |  | WPL (a)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
401(k) costs |  | $4.7 |  |  |  | $4.4 |  |  |  | $14.7 |  |  |  | $14.1 |  |  |  | $2.4 |  |  |  | $2.4 |  |  |  | $7.6 |  |  |  | $7.4 |  |  |  | $2.1 |  |  |  | $2.0 |  |  |  | $6.5 |  |  |  | $6.2 |
(a) | IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense 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
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Compensation expense |  | $4.0 |  |  |  | $0.1 |  |  |  | $8.6 |  |  |  | $3.8 |  |  |  | $2.0 |  |  |  | $0.1 |  |  |  | $4.4 |  |  |  | $2.0 |  |  |  | $1.7 |  |  |  | $0.1 |  |  |  | $3.7 |  |  |  | $1.7 |
Income tax benefits | 1.6 |  |  | — |  |  | 3.5 |  |  | 1.5 |  |  | 0.8 |  |  | — |  |  | 1.8 |  |  | 0.8 |  |  | 0.7 |  |  | 0.1 |  |  | 1.5 |  |  | 0.7 |
Schedule Of Equity Incentive Plans Activity | Information related to fair values of nonvested performance shares and units at September 30, 2013 by year of grant were as follows:
| Performance Shares |  | Performance Units
| 2013 Grant |  | 2012 Grant |  | 2011 Grant |  | 2013 Grant |  | 2012 Grant |  | 2011 Grant
Nonvested awards | 49,093 |  |  | 45,612 |  |  | 45,235 |  |  | 21,935 |  |  | 23,226 |  |  | 20,751 |
Alliant Energy common stock closing price on September 30, 2013 |  | $49.55 |  |  |  | $49.55 |  |  |  | $49.55 |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date |  |  |  |  |  |  |  | $47.58 |  |  |  | $43.05 |  |  |  | $38.75 |
Estimated payout percentage based on performance criteria | 103 | % |  | 100 | % |  | 130 | % |  | 103 | % |  | 100 | % |  | 130 | %
Fair values of each nonvested award |  | $51.04 |  |  |  | $49.55 |  |  |  | $64.42 |  |  |  | $49.01 |  |  |  | $43.05 |  |  |  | $50.38 |
Performance Contingent Restricted Stock [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | A summary of the performance-contingent restricted stock activity was as follows:
| 2013 |  | 2012
| Shares |  | Weighted Average Fair Value |  | Shares |  | Weighted Average Fair Value
Nonvested shares, January 1 | 211,651 |  |  |  | $32.42 |  |  | 301,738 |  |  |  | $32.60 |
Granted | 49,093 |  |  | 47.58 |  |  | 45,612 |  |  | 43.05 |
Vested (a) | — |  |  | — |  |  | (65,172 | ) |  | 32.56 |
Forfeited (b) | (101,822 | ) |  | 23.67 |  |  | (70,527 | ) |  | 39.93 |
Nonvested shares, September 30 | 158,922 |  |  | 42.71 |  |  | 211,651 |  |  | 32.42 |
(a) | In 2012, 65,172 performance-contingent restricted shares granted in 2010 vested because the specified performance criteria for such shares were met.
(b) | In 2013 and 2012, 101,822 and 65,516 performance-contingent restricted shares granted in 2009 and 2008, respectively, were forfeited because the specified performance criteria for such shares were not met. The remaining forfeitures during 2012 were primarily caused by retirements and terminations of participants.
Performance Contingent Cash Awards [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | A summary of the performance contingent cash awards activity was as follows:
| 2013 |  | 2012
| Awards |  | Awards
Nonvested awards, January 1 | 59,639 |  |  | 46,676 |
Granted | 39,530 |  |  | 36,936 |
Vested (a) | — |  |  | (21,605 | )
Forfeited | (1,413 | ) |  | (1,533 | )
Nonvested awards, September 30 | 97,756 |  |  | 60,474 |
(a) | In 2012, 21,605 performance contingent cash awards granted in 2010 vested, resulting in cash payouts valued at $0.9 million.
Performance Shares [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Schedule Of Equity Incentive Plans Activity | A summary of the performance shares activity was as follows:
| 2013 |  | 2012
| Shares (a) |  | Shares (a)
Nonvested shares, January 1 | 145,277 |  |  | 236,979 |
Granted | 49,093 |  |  | 45,612 |
Vested (b) | (54,430 | ) |  | (111,980 | )
Forfeited | — |  |  | (25,334 | )
Nonvested shares, September 30 | 139,940 |  |  | 145,277 |
(a) | Share amounts represent the target number of performance shares. Each performance share’s value is based on the price of one share of Alliant Energy’s common stock at the end of the performance period. The actual number of shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares.
(b) | In 2013, 54,430 performance shares granted in 2010 vested at 197.5% of the target, resulting in payouts valued at $4.8 million, which consisted of a combination of cash and common stock (4,177 shares). In 2012, 111,980 performance shares granted in 2009 vested at 162.5% of the target, resulting in payouts valued at $8.0 million, which consisted of a combination of cash and common stock (6,399 shares).
Performance Units [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Defined Benefit Pension And Other Postretirement Benefits Plans | A summary of the performance unit activity was as follows:
| 2013 |  | 2012
| Units (a) |  | Units (a)
Nonvested units, January 1 | 64,969 |  |  | 42,996 |
Granted | 22,201 |  |  | 24,686 |
Vested (b) | (19,760 | ) |  | — |
Forfeited | (1,498 | ) |  | (878 | )
Nonvested units, September 30 | 65,912 |  |  | 66,804 |
(a) | Unit amounts represent the target number of performance units. Each performance unit’s value is based on the closing price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance units is dependent upon actual performance and may range from zero to 200% of the target number of units.
(b) | In 2013, 19,760 performance units granted in 2010 vested at 197.5% of the target, resulting in cash payouts valued at $1.3 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 Alliant Energy’s, IPL’s and WPL’s sponsored defined benefit pension and other postretirement benefits 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 IPL’s and WPL’s 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 other postretirement benefits plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively.
Alliant Energy | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $3.9 |  |  |  | $3.3 |  |  |  | $11.8 |  |  |  | $10.0 |  |  |  | $1.5 |  |  |  | $1.7 |  |  |  | $4.7 |  |  |  | $5.2 |
Interest cost | 12.3 |  |  | 13.0 |  |  | 36.8 |  |  | 38.9 |  |  | 2.1 |  |  | 2.6 |  |  | 6.3 |  |  | 7.7 |
Expected return on plan assets | (18.5 | ) |  | (17.2 | ) |  | (55.5 | ) |  | (51.6 | ) |  | (2.0 | ) |  | (1.9 | ) |  | (6.0 | ) |  | (5.7 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (3.0 | ) |  | (3.0 | ) |  | (8.9 | ) |  | (9.0 | )
Amortization of actuarial loss | 9.0 |  |  | 8.3 |  |  | 27.1 |  |  | 24.9 |  |  | 1.3 |  |  | 1.6 |  |  | 3.7 |  |  | 4.7 |
|  | $6.8 |  |  |  | $7.5 |  |  |  | $20.4 |  |  |  | $22.4 |  |  |  | ($0.1 | ) |  |  | $1.0 |  |  |  | ($0.2 | ) |  |  | $2.9 |
IPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.5 |  |  |  | $5.6 |  |  |  | $0.8 |  |  |  | $0.7 |  |  |  | $2.2 |  |  |  | $2.2 |
Interest cost | 5.7 |  |  | 6.1 |  |  | 17.1 |  |  | 18.1 |  |  | 0.9 |  |  | 1.1 |  |  | 2.7 |  |  | 3.3 |
Expected return on plan assets | (8.8 | ) |  | (8.2 | ) |  | (26.4 | ) |  | (24.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (4.2 | ) |  | (3.9 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (1.6 | ) |  | (1.5 | ) |  | (4.7 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 3.6 |  |  | 11.4 |  |  | 10.6 |  |  | 0.6 |  |  | 0.9 |  |  | 2.0 |  |  | 2.7 |
|  | $2.9 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $9.9 |  |  |  | ($0.7 | ) |  |  | ($0.1 | ) |  |  | ($2.0 | ) |  |  | ($0.4 | )
WPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $1.5 |  |  |  | $1.3 |  |  |  | $4.4 |  |  |  | $3.9 |  |  |  | $0.6 |  |  |  | $0.7 |  |  |  | $1.8 |  |  |  | $2.1 |
Interest cost | 5.2 |  |  | 5.4 |  |  | 15.5 |  |  | 16.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 3.1 |
Expected return on plan assets | (8.0 | ) |  | (7.5 | ) |  | (23.9 | ) |  | (22.3 | ) |  | (0.4 | ) |  | (0.3 | ) |  | (1.0 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.2 |  |  | 0.3 |  |  | 0.3 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.9 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 3.9 |  |  | 12.8 |  |  | 11.8 |  |  | 0.5 |  |  | 0.6 |  |  | 1.5 |  |  | 1.7 |
|  | $3.0 |  |  |  | $3.3 |  |  |  | $9.1 |  |  |  | $9.9 |  |  |  | $0.6 |  |  |  | $1.0 |  |  |  | $1.9 |  |  |  | $3.0 |
Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs | The following table includes the allocated qualified and non-qualified pension and other postretirement benefits costs (credits) associated with Corporate Services employees providing services to IPL and WPL for the three and nine months ended September 30 (in millions):
| Pension Benefits Costs |  | Other Postretirement Benefits Costs (Credits)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
IPL |  | $0.4 |  |  |  | $0.4 |  |  |  | $1.4 |  |  |  | $1.4 |  |  |  | ($0.1 | ) |  |  | $— |  |  |  | ($0.2 | ) |  |  | $0.1 |
WPL | 0.4 |  |  | 0.4 |  |  | 1.1 |  |  | 1.0 |  |  | (0.1 | ) |  | 0.1 |  |  | (0.2 | ) |  | 0.1 |
Estimated Future And Actual Employer Contributions | Estimated and actual funding for the qualified and non-qualified defined benefit pension and other postretirement benefits plans for 2013 are as follows (in millions):
| Estimated for Calendar Year 2013 |  | Actual Through September 30, 2013
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $2.3 |  |  |  | $0.8 |  |  |  | $0.2 |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $0.2 |
Other postretirement benefits plans | 3.0 |  |  | — |  |  | 3.0 |  |  | 2.0 |  |  | — |  |  | 2.0 |
(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.
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’ level of contribution, were as follows (in millions):
| Alliant Energy |  | IPL (a) |  | WPL (a)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
401(k) costs |  | $4.7 |  |  |  | $4.4 |  |  |  | $14.7 |  |  |  | $14.1 |  |  |  | $2.4 |  |  |  | $2.4 |  |  |  | $7.6 |  |  |  | $7.4 |  |  |  | $2.1 |  |  |  | $2.0 |  |  |  | $6.5 |  |  |  | $6.2 |
(a) | IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense 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
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Compensation expense |  | $4.0 |  |  |  | $0.1 |  |  |  | $8.6 |  |  |  | $3.8 |  |  |  | $2.0 |  |  |  | $0.1 |  |  |  | $4.4 |  |  |  | $2.0 |  |  |  | $1.7 |  |  |  | $0.1 |  |  |  | $3.7 |  |  |  | $1.7 |
Income tax benefits | 1.6 |  |  | — |  |  | 3.5 |  |  | 1.5 |  |  | 0.8 |  |  | — |  |  | 1.8 |  |  | 0.8 |  |  | 0.7 |  |  | 0.1 |  |  | 1.5 |  |  | 0.7 |
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 Alliant Energy’s, IPL’s and WPL’s sponsored defined benefit pension and other postretirement benefits 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 IPL’s and WPL’s 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 other postretirement benefits plans costs (credits) represent costs (credits) for IPL and WPL employees, respectively.
Alliant Energy | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $3.9 |  |  |  | $3.3 |  |  |  | $11.8 |  |  |  | $10.0 |  |  |  | $1.5 |  |  |  | $1.7 |  |  |  | $4.7 |  |  |  | $5.2 |
Interest cost | 12.3 |  |  | 13.0 |  |  | 36.8 |  |  | 38.9 |  |  | 2.1 |  |  | 2.6 |  |  | 6.3 |  |  | 7.7 |
Expected return on plan assets | (18.5 | ) |  | (17.2 | ) |  | (55.5 | ) |  | (51.6 | ) |  | (2.0 | ) |  | (1.9 | ) |  | (6.0 | ) |  | (5.7 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.1 |  |  | 0.2 |  |  | 0.2 |  |  | (3.0 | ) |  | (3.0 | ) |  | (8.9 | ) |  | (9.0 | )
Amortization of actuarial loss | 9.0 |  |  | 8.3 |  |  | 27.1 |  |  | 24.9 |  |  | 1.3 |  |  | 1.6 |  |  | 3.7 |  |  | 4.7 |
|  | $6.8 |  |  |  | $7.5 |  |  |  | $20.4 |  |  |  | $22.4 |  |  |  | ($0.1 | ) |  |  | $1.0 |  |  |  | ($0.2 | ) |  |  | $2.9 |
IPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $2.2 |  |  |  | $1.8 |  |  |  | $6.5 |  |  |  | $5.6 |  |  |  | $0.8 |  |  |  | $0.7 |  |  |  | $2.2 |  |  |  | $2.2 |
Interest cost | 5.7 |  |  | 6.1 |  |  | 17.1 |  |  | 18.1 |  |  | 0.9 |  |  | 1.1 |  |  | 2.7 |  |  | 3.3 |
Expected return on plan assets | (8.8 | ) |  | (8.2 | ) |  | (26.4 | ) |  | (24.5 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (4.2 | ) |  | (3.9 | )
Amortization of prior service cost (credit) | — |  |  | — |  |  | 0.1 |  |  | 0.1 |  |  | (1.6 | ) |  | (1.5 | ) |  | (4.7 | ) |  | (4.7 | )
Amortization of actuarial loss | 3.8 |  |  | 3.6 |  |  | 11.4 |  |  | 10.6 |  |  | 0.6 |  |  | 0.9 |  |  | 2.0 |  |  | 2.7 |
|  | $2.9 |  |  |  | $3.3 |  |  |  | $8.7 |  |  |  | $9.9 |  |  |  | ($0.7 | ) |  |  | ($0.1 | ) |  |  | ($2.0 | ) |  |  | ($0.4 | )
WPL | Defined Benefit Pension Plans |  | Other Postretirement Benefits Plans
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Service cost |  | $1.5 |  |  |  | $1.3 |  |  |  | $4.4 |  |  |  | $3.9 |  |  |  | $0.6 |  |  |  | $0.7 |  |  |  | $1.8 |  |  |  | $2.1 |
Interest cost | 5.2 |  |  | 5.4 |  |  | 15.5 |  |  | 16.2 |  |  | 0.8 |  |  | 1.0 |  |  | 2.5 |  |  | 3.1 |
Expected return on plan assets | (8.0 | ) |  | (7.5 | ) |  | (23.9 | ) |  | (22.3 | ) |  | (0.4 | ) |  | (0.3 | ) |  | (1.0 | ) |  | (1.0 | )
Amortization of prior service cost (credit) | 0.1 |  |  | 0.2 |  |  | 0.3 |  |  | 0.3 |  |  | (0.9 | ) |  | (1.0 | ) |  | (2.9 | ) |  | (2.9 | )
Amortization of actuarial loss | 4.2 |  |  | 3.9 |  |  | 12.8 |  |  | 11.8 |  |  | 0.5 |  |  | 0.6 |  |  | 1.5 |  |  | 1.7 |
|  | $3.0 |  |  |  | $3.3 |  |  |  | $9.1 |  |  |  | $9.9 |  |  |  | $0.6 |  |  |  | $1.0 |  |  |  | $1.9 |  |  |  | $3.0 |
Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs | The following table includes the allocated qualified and non-qualified pension and other postretirement benefits costs (credits) associated with Corporate Services employees providing services to IPL and WPL for the three and nine months ended September 30 (in millions):
| Pension Benefits Costs |  | Other Postretirement Benefits Costs (Credits)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
IPL |  | $0.4 |  |  |  | $0.4 |  |  |  | $1.4 |  |  |  | $1.4 |  |  |  | ($0.1 | ) |  |  | $— |  |  |  | ($0.2 | ) |  |  | $0.1 |
WPL | 0.4 |  |  | 0.4 |  |  | 1.1 |  |  | 1.0 |  |  | (0.1 | ) |  | 0.1 |  |  | (0.2 | ) |  | 0.1 |
Estimated Future And Actual Employer Contributions | Estimated and actual funding for the qualified and non-qualified defined benefit pension and other postretirement benefits plans for 2013 are as follows (in millions):
| Estimated for Calendar Year 2013 |  | Actual Through September 30, 2013
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Defined benefit pension plans (a) |  | $2.3 |  |  |  | $0.8 |  |  |  | $0.2 |  |  |  | $2.0 |  |  |  | $0.7 |  |  |  | $0.2 |
Other postretirement benefits plans | 3.0 |  |  | — |  |  | 3.0 |  |  | 2.0 |  |  | — |  |  | 2.0 |
(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.
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’ level of contribution, were as follows (in millions):
| Alliant Energy |  | IPL (a) |  | WPL (a)
| Three Months |  | Nine Months |  | Three Months |  | Nine Months |  | Three Months |  | Nine Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
401(k) costs |  | $4.7 |  |  |  | $4.4 |  |  |  | $14.7 |  |  |  | $14.1 |  |  |  | $2.4 |  |  |  | $2.4 |  |  |  | $7.6 |  |  |  | $7.4 |  |  |  | $2.1 |  |  |  | $2.0 |  |  |  | $6.5 |  |  |  | $6.2 |
(a) | IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.
Recognized Compensation Expense And Income Tax Benefits | A summary of compensation expense 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
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
Compensation expense |  | $4.0 |  |  |  | $0.1 |  |  |  | $8.6 |  |  |  | $3.8 |  |  |  | $2.0 |  |  |  | $0.1 |  |  |  | $4.4 |  |  |  | $2.0 |  |  |  | $1.7 |  |  |  | $0.1 |  |  |  | $3.7 |  |  |  | $1.7 |
Income tax benefits | 1.6 |  |  | — |  |  | 3.5 |  |  | 1.5 |  |  | 0.8 |  |  | — |  |  | 1.8 |  |  | 0.8 |  |  | 0.7 |  |  | 0.1 |  |  | 1.5 |  |  | 0.7 |

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