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Regulatory Matters (Regulatory Assets) (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Regulatory Assets [Line Items]    
Regulatory assets $ 2,016.0 $ 1,915.1
Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 1,100.7 1,055.6
Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 558.3 578.7
Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 108.7 105.9
EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 78.2 41.4
Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 47.9 30.7
Emission allowances [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 25.9 26.2
Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 96.3 76.6
IPL [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 1,556.6 1,458.8
IPL [Member] | Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 1,063.3 1,022.4
IPL [Member] | Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 284.2 294.0
IPL [Member] | Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 72.5 64.3
IPL [Member] | EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 40.3 0.0
IPL [Member] | Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 18.1 10.0
IPL [Member] | Emission allowances [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 25.9 26.2
IPL [Member] | Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 52.3 41.9
WPL [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 459.4 456.3
WPL [Member] | Tax-related [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 37.4 33.2
WPL [Member] | Pension and OPEB costs [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 274.1 284.7
WPL [Member] | Asset retirement obligations [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 36.2 41.6
WPL [Member] | EGUs retired early [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 37.9 41.4
WPL [Member] | Derivatives [Member]    
Regulatory Assets [Line Items]    
Regulatory assets 29.8 20.7
WPL [Member] | Other [Member]    
Regulatory Assets [Line Items]    
Regulatory assets $ 44.0 $ 34.7