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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 2,010 | $ 1,845
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 890 | 818
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 580 | 524
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 119 | 112
Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 113 | 134
IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 110 | 108
WPL's Western Wisconsin gas distribution expansion investments [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 55 | 0
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26 | 39
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 117 | 110
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,483 | 1,400
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 843 | 777
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 291 | 263
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 81 | 76
IPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 77 | 88
IPL [Member] | IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 110 | 108
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13 | 18
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 68 | 70
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 527 | 445
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 47 | 41
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 289 | 261
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 38 | 36
WPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 36 | 46
WPL [Member] | WPL's Western Wisconsin gas distribution expansion investments [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 55 | 0
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13 | 21
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 49 | $ 40

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