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Benefit Plans (Tables) | 6 Months Ended
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Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | 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 those for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain 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 | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $2.7 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $4.8 |  |  |  | $0.9 |  |  |  | $0.9 |  |  |  | $1.7 |  |  |  | $1.7 |
Interest cost | 10.8 |  |  | 12.4 |  |  | 21.6 |  |  | 24.9 |  |  | 1.7 |  |  | 2.1 |  |  | 3.5 |  |  | 4.2 |
Expected return on plan assets | (17.5 | ) |  | (15.0 | ) |  | (34.9 | ) |  | (30.0 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (2.7 | ) |  | (2.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 8.6 |  |  | 9.1 |  |  | 17.2 |  |  | 18.2 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Settlement losses (a) | — |  |  | — |  |  | 4.2 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.5 |  |  |  | $8.8 |  |  |  | $13.2 |  |  |  | $17.6 |  |  |  | $2.1 |  |  |  | $2.4 |  |  |  | $4.2 |  |  |  | $4.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $1.7 |  |  |  | $1.5 |  |  |  | $3.4 |  |  |  | $3.0 |  |  |  | $0.4 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |
Interest cost | 5.0 |  |  | 5.7 |  |  | 10.0 |  |  | 11.4 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (8.2 | ) |  | (7.1 | ) |  | (16.3 | ) |  | (14.1 | ) |  | (0.9 | ) |  | (0.9 | ) |  | (1.9 | ) |  | (1.8 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 7.4 |  |  | 7.9 |  |  | 0.2 |  |  | 0.3 |  |  | 0.5 |  |  | 0.7 |
|  | $2.2 |  |  |  | $4.1 |  |  |  | $4.4 |  |  |  | $8.1 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |  |  |  | $1.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $0.9 |  |  |  | $0.8 |  |  |  | $1.9 |  |  |  | $1.7 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $0.6 |  |  |  | $0.6 |
Interest cost | 4.7 |  |  | 5.4 |  |  | 9.4 |  |  | 10.8 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (7.6 | ) |  | (6.6 | ) |  | (15.2 | ) |  | (13.1 | ) |  | (0.1 | ) |  | (0.2 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 4.1 |  |  | 4.4 |  |  | 8.2 |  |  | 8.8 |  |  | 0.5 |  |  | 0.4 |  |  | 1.0 |  |  | 0.8 |
|  | $2.1 |  |  |  | $4.0 |  |  |  | $4.2 |  |  |  | $8.1 |  |  |  | $1.4 |  |  |  | $1.3 |  |  |  | $2.7 |  |  |  | $2.7 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Compensation expense |  | $2.7 |  |  |  | $3.4 |  |  |  | $5.4 |  |  |  | $8.1 |  |  |  | $1.5 |  |  |  | $1.9 |  |  |  | $2.9 |  |  |  | $4.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $2.3 |  |  |  | $3.2 |
Income tax benefits | 0.8 |  |  | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.3 |  |  | 0.3 |  |  | 0.4 |  |  | 0.6 |  |  | 0.9 |
Schedule Of Equity-based Compensation Plans Activity | For the six months ended June 30, 2020, performance shares, performance restricted stock units and restricted stock units were granted to key employees as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards. As of June 30, 2020, 228,207 shares were included in the calculation of diluted EPS related to the nonvested equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares | 55,303 |  |  |  | $64.04 |
Performance restricted stock units | 55,303 |  |  | 59.47 |
Restricted stock units | 60,284 |  |  | 59.49 |
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | 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 those for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain 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 | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $2.7 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $4.8 |  |  |  | $0.9 |  |  |  | $0.9 |  |  |  | $1.7 |  |  |  | $1.7 |
Interest cost | 10.8 |  |  | 12.4 |  |  | 21.6 |  |  | 24.9 |  |  | 1.7 |  |  | 2.1 |  |  | 3.5 |  |  | 4.2 |
Expected return on plan assets | (17.5 | ) |  | (15.0 | ) |  | (34.9 | ) |  | (30.0 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (2.7 | ) |  | (2.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 8.6 |  |  | 9.1 |  |  | 17.2 |  |  | 18.2 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Settlement losses (a) | — |  |  | — |  |  | 4.2 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.5 |  |  |  | $8.8 |  |  |  | $13.2 |  |  |  | $17.6 |  |  |  | $2.1 |  |  |  | $2.4 |  |  |  | $4.2 |  |  |  | $4.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $1.7 |  |  |  | $1.5 |  |  |  | $3.4 |  |  |  | $3.0 |  |  |  | $0.4 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |
Interest cost | 5.0 |  |  | 5.7 |  |  | 10.0 |  |  | 11.4 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (8.2 | ) |  | (7.1 | ) |  | (16.3 | ) |  | (14.1 | ) |  | (0.9 | ) |  | (0.9 | ) |  | (1.9 | ) |  | (1.8 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 7.4 |  |  | 7.9 |  |  | 0.2 |  |  | 0.3 |  |  | 0.5 |  |  | 0.7 |
|  | $2.2 |  |  |  | $4.1 |  |  |  | $4.4 |  |  |  | $8.1 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |  |  |  | $1.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $0.9 |  |  |  | $0.8 |  |  |  | $1.9 |  |  |  | $1.7 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $0.6 |  |  |  | $0.6 |
Interest cost | 4.7 |  |  | 5.4 |  |  | 9.4 |  |  | 10.8 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (7.6 | ) |  | (6.6 | ) |  | (15.2 | ) |  | (13.1 | ) |  | (0.1 | ) |  | (0.2 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 4.1 |  |  | 4.4 |  |  | 8.2 |  |  | 8.8 |  |  | 0.5 |  |  | 0.4 |  |  | 1.0 |  |  | 0.8 |
|  | $2.1 |  |  |  | $4.0 |  |  |  | $4.2 |  |  |  | $8.1 |  |  |  | $1.4 |  |  |  | $1.3 |  |  |  | $2.7 |  |  |  | $2.7 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Compensation expense |  | $2.7 |  |  |  | $3.4 |  |  |  | $5.4 |  |  |  | $8.1 |  |  |  | $1.5 |  |  |  | $1.9 |  |  |  | $2.9 |  |  |  | $4.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $2.3 |  |  |  | $3.2 |
Income tax benefits | 0.8 |  |  | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.3 |  |  | 0.3 |  |  | 0.4 |  |  | 0.6 |  |  | 0.9 |
Schedule Of Equity-based Compensation Plans Activity | For the six months ended June 30, 2020, performance shares, performance restricted stock units and restricted stock units were granted to key employees as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards. As of June 30, 2020, 228,207 shares were included in the calculation of diluted EPS related to the nonvested equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares | 55,303 |  |  |  | $64.04 |
Performance restricted stock units | 55,303 |  |  | 59.47 |
Restricted stock units | 60,284 |  |  | 59.49 |
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Net Periodic Benefit Costs | 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 those for their plan participants covered under plans they sponsor, as well as amounts directly assigned to them related to certain 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 | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $2.7 |  |  |  | $2.4 |  |  |  | $5.4 |  |  |  | $4.8 |  |  |  | $0.9 |  |  |  | $0.9 |  |  |  | $1.7 |  |  |  | $1.7 |
Interest cost | 10.8 |  |  | 12.4 |  |  | 21.6 |  |  | 24.9 |  |  | 1.7 |  |  | 2.1 |  |  | 3.5 |  |  | 4.2 |
Expected return on plan assets | (17.5 | ) |  | (15.0 | ) |  | (34.9 | ) |  | (30.0 | ) |  | (1.4 | ) |  | (1.3 | ) |  | (2.7 | ) |  | (2.5 | )
Amortization of prior service credit | (0.1 | ) |  | (0.1 | ) |  | (0.3 | ) |  | (0.3 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 8.6 |  |  | 9.1 |  |  | 17.2 |  |  | 18.2 |  |  | 0.9 |  |  | 0.8 |  |  | 1.7 |  |  | 1.6 |
Settlement losses (a) | — |  |  | — |  |  | 4.2 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
|  | $4.5 |  |  |  | $8.8 |  |  |  | $13.2 |  |  |  | $17.6 |  |  |  | $2.1 |  |  |  | $2.4 |  |  |  | $4.2 |  |  |  | $4.9 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
IPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $1.7 |  |  |  | $1.5 |  |  |  | $3.4 |  |  |  | $3.0 |  |  |  | $0.4 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |
Interest cost | 5.0 |  |  | 5.7 |  |  | 10.0 |  |  | 11.4 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (8.2 | ) |  | (7.1 | ) |  | (16.3 | ) |  | (14.1 | ) |  | (0.9 | ) |  | (0.9 | ) |  | (1.9 | ) |  | (1.8 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |
Amortization of actuarial loss | 3.7 |  |  | 4.0 |  |  | 7.4 |  |  | 7.9 |  |  | 0.2 |  |  | 0.3 |  |  | 0.5 |  |  | 0.7 |
|  | $2.2 |  |  |  | $4.1 |  |  |  | $4.4 |  |  |  | $8.1 |  |  |  | $0.4 |  |  |  | $0.7 |  |  |  | $0.7 |  |  |  | $1.3 |
| Defined Benefit Pension Plans |  | OPEB Plans
| Three Months |  | Six Months |  | Three Months |  | Six Months
WPL | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost |  | $0.9 |  |  |  | $0.8 |  |  |  | $1.9 |  |  |  | $1.7 |  |  |  | $0.3 |  |  |  | $0.3 |  |  |  | $0.6 |  |  |  | $0.6 |
Interest cost | 4.7 |  |  | 5.4 |  |  | 9.4 |  |  | 10.8 |  |  | 0.7 |  |  | 0.9 |  |  | 1.4 |  |  | 1.7 |
Expected return on plan assets | (7.6 | ) |  | (6.6 | ) |  | (15.2 | ) |  | (13.1 | ) |  | (0.1 | ) |  | (0.2 | ) |  | (0.3 | ) |  | (0.3 | )
Amortization of prior service credit | — |  |  | — |  |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | (0.1 | ) |  | — |  |  | (0.1 | )
Amortization of actuarial loss | 4.1 |  |  | 4.4 |  |  | 8.2 |  |  | 8.8 |  |  | 0.5 |  |  | 0.4 |  |  | 1.0 |  |  | 0.8 |
|  | $2.1 |  |  |  | $4.0 |  |  |  | $4.2 |  |  |  | $8.1 |  |  |  | $1.4 |  |  |  | $1.3 |  |  |  | $2.7 |  |  |  | $2.7 |
(a) | Settlement losses related to payments made to retired executives of Alliant Energy.
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
| 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Compensation expense |  | $2.7 |  |  |  | $3.4 |  |  |  | $5.4 |  |  |  | $8.1 |  |  |  | $1.5 |  |  |  | $1.9 |  |  |  | $2.9 |  |  |  | $4.5 |  |  |  | $1.2 |  |  |  | $1.4 |  |  |  | $2.3 |  |  |  | $3.2 |
Income tax benefits | 0.8 |  |  | 1.0 |  |  | 1.5 |  |  | 2.3 |  |  | 0.4 |  |  | 0.5 |  |  | 0.8 |  |  | 1.3 |  |  | 0.3 |  |  | 0.4 |  |  | 0.6 |  |  | 0.9 |
Schedule Of Equity-based Compensation Plans Activity | For the six months ended June 30, 2020, performance shares, performance restricted stock units and restricted stock units were granted to key employees as follows. These shares and units will be paid out in shares of common stock, and are therefore accounted for as equity awards. As of June 30, 2020, 228,207 shares were included in the calculation of diluted EPS related to the nonvested equity awards.
|  |  | Weighted Average
| Grants |  | Grant Date Fair Value
Performance shares | 55,303 |  |  |  | $64.04 |
Performance restricted stock units | 55,303 |  |  | 59.47 |
Restricted stock units | 60,284 |  |  | 59.49 |

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