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Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | $ 1,305 | $ 1,234
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 1,305 | 1,234
Fair value of plan assets | 984 | 931
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 1,351 | 1,280
Fair value of plan assets | 984 | 931
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 227 | 214
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 227 | 214
Fair value of plan assets | 108 | 105
IPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 589 | 558
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 589 | 558
Fair value of plan assets | 453 | 436
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 615 | 581
Fair value of plan assets | 453 | 436
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 91 | 86
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 91 | 86
Fair value of plan assets | 74 | 72
WPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 573 | 536
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 573 | 536
Fair value of plan assets | 436 | 406
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 588 | 550
Fair value of plan assets | 436 | 406
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 89 | 85
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 89 | 85
Fair value of plan assets | $ 18 | $ 17

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