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Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2019
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 and six months ended June 30 are included below (in millions). For IPL and WPL, amounts represent their current and former employees covered under plans they sponsor, as well as amounts directly assigned to them related to their current and former employees who are participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $2.4 |  |  |  | $3.0 |  |  |  | $4.8 |  |  |  | $6.0 |  |  |  | $0.9 |  |  |  | $1.0 |  |  |  | $1.7 |  |  |  | $2.1 |
Interest cost | 12.4 |  |  | 11.7 |  |  | 24.9 |  |  | 23.4 |  |  | 2.1 |  |  | 1.9 |  |  | 4.2 |  |  | 3.8 |
Expected return on plan assets | (15.0 | ) |  | (17.5 | ) |  | (30.0 | ) |  | (34.9 | ) |  | (1.3 | ) |  | (1.5 | ) |  | (2.5 | ) |  | (3.0 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 9.1 |  |  | 8.8 |  |  | 18.2 |  |  | 17.6 |  |  | 0.8 |  |  | 0.9 |  |  | 1.6 |  |  | 1.7 |
|  | $8.8 |  |  |  | $5.9 |  |  |  | $17.6 |  |  |  | $11.8 |  |  |  | $2.4 |  |  |  | $2.2 |  |  |  | $4.9 |  |  |  | $4.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $1.5 |  |  |  | $1.9 |  |  |  | $3.0 |  |  |  | $3.7 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $0.7 |  |  |  | $0.9 |
Interest cost | 5.7 |  |  | 5.4 |  |  | 11.4 |  |  | 10.7 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Expected return on plan assets | (7.1 | ) |  | (8.2 | ) |  | (14.1 | ) |  | (16.3 | ) |  | (0.9 | ) |  | (1.1 | ) |  | (1.8 | ) |  | (2.2 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 4.0 |  |  | 3.8 |  |  | 7.9 |  |  | 7.5 |  |  | 0.3 |  |  | 0.3 |  |  | 0.7 |  |  | 0.6 |
|  | $4.1 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.5 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $0.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $0.8 |  |  |  | $1.1 |  |  |  | $1.7 |  |  |  | $2.2 |  |  |  | $0.3 |  |  |  | $0.4 |  |  |  | $0.6 |  |  |  | $0.8 |
Interest cost | 5.4 |  |  | 5.1 |  |  | 10.8 |  |  | 10.1 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 1.5 |
Expected return on plan assets | (6.6 | ) |  | (7.6 | ) |  | (13.1 | ) |  | (15.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 4.4 |  |  | 4.3 |  |  | 8.8 |  |  | 8.6 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.0 |
|  | $4.0 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.6 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.7 |  |  |  | $2.9 |
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
| 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Compensation expense |  | $3.4 |  |  |  | $5.1 |  |  |  | $8.1 |  |  |  | $8.4 |  |  |  | $1.9 |  |  |  | $2.8 |  |  |  | $4.5 |  |  |  | $4.6 |  |  |  | $1.4 |  |  |  | $2.1 |  |  |  | $3.2 |  |  |  | $3.4 |
Income tax benefits | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 2.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.3 |  |  | 1.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 0.9 |
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 and six months ended June 30 are included below (in millions). For IPL and WPL, amounts represent their current and former employees covered under plans they sponsor, as well as amounts directly assigned to them related to their current and former employees who are participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $2.4 |  |  |  | $3.0 |  |  |  | $4.8 |  |  |  | $6.0 |  |  |  | $0.9 |  |  |  | $1.0 |  |  |  | $1.7 |  |  |  | $2.1 |
Interest cost | 12.4 |  |  | 11.7 |  |  | 24.9 |  |  | 23.4 |  |  | 2.1 |  |  | 1.9 |  |  | 4.2 |  |  | 3.8 |
Expected return on plan assets | (15.0 | ) |  | (17.5 | ) |  | (30.0 | ) |  | (34.9 | ) |  | (1.3 | ) |  | (1.5 | ) |  | (2.5 | ) |  | (3.0 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 9.1 |  |  | 8.8 |  |  | 18.2 |  |  | 17.6 |  |  | 0.8 |  |  | 0.9 |  |  | 1.6 |  |  | 1.7 |
|  | $8.8 |  |  |  | $5.9 |  |  |  | $17.6 |  |  |  | $11.8 |  |  |  | $2.4 |  |  |  | $2.2 |  |  |  | $4.9 |  |  |  | $4.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $1.5 |  |  |  | $1.9 |  |  |  | $3.0 |  |  |  | $3.7 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $0.7 |  |  |  | $0.9 |
Interest cost | 5.7 |  |  | 5.4 |  |  | 11.4 |  |  | 10.7 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Expected return on plan assets | (7.1 | ) |  | (8.2 | ) |  | (14.1 | ) |  | (16.3 | ) |  | (0.9 | ) |  | (1.1 | ) |  | (1.8 | ) |  | (2.2 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 4.0 |  |  | 3.8 |  |  | 7.9 |  |  | 7.5 |  |  | 0.3 |  |  | 0.3 |  |  | 0.7 |  |  | 0.6 |
|  | $4.1 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.5 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $0.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $0.8 |  |  |  | $1.1 |  |  |  | $1.7 |  |  |  | $2.2 |  |  |  | $0.3 |  |  |  | $0.4 |  |  |  | $0.6 |  |  |  | $0.8 |
Interest cost | 5.4 |  |  | 5.1 |  |  | 10.8 |  |  | 10.1 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 1.5 |
Expected return on plan assets | (6.6 | ) |  | (7.6 | ) |  | (13.1 | ) |  | (15.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 4.4 |  |  | 4.3 |  |  | 8.8 |  |  | 8.6 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.0 |
|  | $4.0 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.6 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.7 |  |  |  | $2.9 |
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
| 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Compensation expense |  | $3.4 |  |  |  | $5.1 |  |  |  | $8.1 |  |  |  | $8.4 |  |  |  | $1.9 |  |  |  | $2.8 |  |  |  | $4.5 |  |  |  | $4.6 |  |  |  | $1.4 |  |  |  | $2.1 |  |  |  | $3.2 |  |  |  | $3.4 |
Income tax benefits | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 2.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.3 |  |  | 1.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 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 and six months ended June 30 are included below (in millions). For IPL and WPL, amounts represent their current and former employees covered under plans they sponsor, as well as amounts directly assigned to them related to their current and former employees who are participants in the Alliant Energy and Corporate Services sponsored plans.
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
Alliant Energy | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $2.4 |  |  |  | $3.0 |  |  |  | $4.8 |  |  |  | $6.0 |  |  |  | $0.9 |  |  |  | $1.0 |  |  |  | $1.7 |  |  |  | $2.1 |
Interest cost | 12.4 |  |  | 11.7 |  |  | 24.9 |  |  | 23.4 |  |  | 2.1 |  |  | 1.9 |  |  | 4.2 |  |  | 3.8 |
Expected return on plan assets | (15.0 | ) |  | (17.5 | ) |  | (30.0 | ) |  | (34.9 | ) |  | (1.3 | ) |  | (1.5 | ) |  | (2.5 | ) |  | (3.0 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 9.1 |  |  | 8.8 |  |  | 18.2 |  |  | 17.6 |  |  | 0.8 |  |  | 0.9 |  |  | 1.6 |  |  | 1.7 |
|  | $8.8 |  |  |  | $5.9 |  |  |  | $17.6 |  |  |  | $11.8 |  |  |  | $2.4 |  |  |  | $2.2 |  |  |  | $4.9 |  |  |  | $4.5 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $1.5 |  |  |  | $1.9 |  |  |  | $3.0 |  |  |  | $3.7 |  |  |  | $0.4 |  |  |  | $0.5 |  |  |  | $0.7 |  |  |  | $0.9 |
Interest cost | 5.7 |  |  | 5.4 |  |  | 11.4 |  |  | 10.7 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Expected return on plan assets | (7.1 | ) |  | (8.2 | ) |  | (14.1 | ) |  | (16.3 | ) |  | (0.9 | ) |  | (1.1 | ) |  | (1.8 | ) |  | (2.2 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 4.0 |  |  | 3.8 |  |  | 7.9 |  |  | 7.5 |  |  | 0.3 |  |  | 0.3 |  |  | 0.7 |  |  | 0.6 |
|  | $4.1 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.5 |  |  |  | $0.7 |  |  |  | $0.5 |  |  |  | $1.3 |  |  |  | $0.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Service cost |  | $0.8 |  |  |  | $1.1 |  |  |  | $1.7 |  |  |  | $2.2 |  |  |  | $0.3 |  |  |  | $0.4 |  |  |  | $0.6 |  |  |  | $0.8 |
Interest cost | 5.4 |  |  | 5.1 |  |  | 10.8 |  |  | 10.1 |  |  | 0.9 |  |  | 0.7 |  |  | 1.7 |  |  | 1.5 |
Expected return on plan assets | (6.6 | ) |  | (7.6 | ) |  | (13.1 | ) |  | (15.2 | ) |  | (0.2 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | ) |  | (0.1 | )
Amortization of actuarial loss | 4.4 |  |  | 4.3 |  |  | 8.8 |  |  | 8.6 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.0 |
|  | $4.0 |  |  |  | $2.8 |  |  |  | $8.1 |  |  |  | $5.6 |  |  |  | $1.3 |  |  |  | $1.4 |  |  |  | $2.7 |  |  |  | $2.9 |
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
| 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018 |  | 2019 |  | 2018
Compensation expense |  | $3.4 |  |  |  | $5.1 |  |  |  | $8.1 |  |  |  | $8.4 |  |  |  | $1.9 |  |  |  | $2.8 |  |  |  | $4.5 |  |  |  | $4.6 |  |  |  | $1.4 |  |  |  | $2.1 |  |  |  | $3.2 |  |  |  | $3.4 |
Income tax benefits | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 2.4 |  |  | 0.5 |  |  | 0.9 |  |  | 1.3 |  |  | 1.4 |  |  | 0.4 |  |  | 0.5 |  |  | 0.9 |  |  | 0.9 |

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