Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 1,854.9	$ 1,844.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	831.2	817.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	510.5	524.0
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	129.2	134.0
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	112.8	111.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.8	108.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.6	39.5
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.5	21.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	90.3	88.5
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,404.9	1,399.3
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	788.3	776.8
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	255.7	262.5
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	85.6	87.9
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.1	76.2
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.8	108.2
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.6	18.3
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.5	21.1
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.3	48.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	450.0	445.4
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.9	40.8
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	254.8	261.5
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.6	46.1
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.7	35.6
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.0	21.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 44.0	$ 40.2