Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory assets	$ 1,806.9	$ 1,737.3
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	815.5	820.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	522.9	542.3
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.0	110.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	107.1	0.0
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	102.8	111.6
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.6	28.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.4	23.6
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	94.6	100.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,364.5	1,279.0
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	780.2	783.1
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	264.7	274.0
IPL [Member] | Asset retirement obligations [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	107.1	0.0
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.7	55.4
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	13.4	15.1
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.4	23.6
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	48.8	51.5
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	442.4	458.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.3	37.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	258.2	268.3
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.8	34.5
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.1	56.2
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.2	12.9
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 45.8	$ 48.9