Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,840.0	$ 1,737.3
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	825.2	820.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	513.1	542.3
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	138.7	111.6
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	110.7	110.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	107.7	0.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.0	28.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.7	23.6
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	88.9	100.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,404.0	1,279.0
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	787.2	783.1
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	260.0	274.0
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	90.1	55.4
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	75.2	76.3
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	107.7	0.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	15.0	15.1
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.7	23.6
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	47.1	51.5
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	436.0	458.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.0	37.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	253.1	268.3
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	48.6	56.2
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.5	34.5
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.0	12.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 41.8	$ 48.9