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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