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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 2,007 | $ 2,010
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 901 | 890
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 570 | 580
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 120 | 119
Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108 | 113
IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108 | 110
WPL's Western Wisconsin gas distribution expansion investments [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 55 | 55
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13 | 26
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 132 | 117
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,494 | 1,483
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 853 | 843
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 286 | 291
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 82 | 81
IPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 74 | 77
IPL [Member] | IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108 | 110
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 6 | 13
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 85 | 68
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 513 | 527
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 48 | 47
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 284 | 289
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 38 | 38
WPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 34 | 36
WPL [Member] | WPL's Western Wisconsin gas distribution expansion investments [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 55 | 55
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 7 | 13
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 47 | $ 49

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