Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,844.7	$ 1,737.3
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	817.6	820.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	524.0	542.3
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	134.0	111.6
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.8	110.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.2	0.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.5	28.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.1	23.6
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	88.5	100.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,399.3	1,279.0
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	776.8	783.1
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	262.5	274.0
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	87.9	55.4
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	76.2	76.3
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.2	0.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.3	15.1
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.1	23.6
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	48.3	51.5
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	445.4	458.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	40.8	37.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	261.5	268.3
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.1	56.2
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.6	34.5
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.2	12.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 40.2	$ 48.9