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Regulatory Matters (Regulatory Assets) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Regulatory Assets [Line Items]    
Regulatory assets $ 1,678.4 $ 1,666.7
Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 776.0 778.2
Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 529.2 548.0
Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 109.8 109.3
EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 91.2 63.8
Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 49.4 45.3
Emission allowances [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 24.9 25.5
Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 97.9 96.6
IPL [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 1,249.2 1,231.6
IPL [Member] | Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 742.2 750.5
IPL [Member] | Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 265.6 274.4
IPL [Member] | Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 76.3 72.5
IPL [Member] | EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 60.2 31.6
IPL [Member] | Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 27.9 21.8
IPL [Member] | Emission allowances [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 24.9 25.5
IPL [Member] | Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 52.1 55.3
WPL [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 429.2 435.1
WPL [Member] | Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 33.8 27.7
WPL [Member] | Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 263.6 273.6
WPL [Member] | Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 33.5 36.8
WPL [Member] | EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 31.0 32.2
WPL [Member] | Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 21.5 23.5
WPL [Member] | Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets $ 45.8 $ 41.3