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Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | $ 1,234.4 | $ 1,139.9
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 1,234.4 | 1,139.9
Fair value of plan assets | 930.4 | 808.6
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 1,279.7 | 1,175.0
Fair value of plan assets | 930.4 | 808.6
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 213.6 | 206.1
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 213.6 | 206.1
Fair value of plan assets | 104.9 | 99.1
IPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 557.8 | 514.3
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 557.8 | 514.3
Fair value of plan assets | 436.0 | 377.8
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 581.2 | 533.9
Fair value of plan assets | 436.0 | 377.8
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 85.7 | 82.5
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 85.7 | 82.5
Fair value of plan assets | 71.9 | 66.7
WPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 535.9 | 494.8
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 535.9 | 494.8
Fair value of plan assets | 406.0 | 352.2
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 550.1 | 506.6
Fair value of plan assets | 406.0 | 352.2
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 85.3 | 83.2
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 85.3 | 83.2
Fair value of plan assets | $ 17.1 | $ 16.7

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