Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,694.7	$ 1,666.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	798.3	778.2
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	519.8	548.0
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.6	109.3
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	110.4	63.8
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.3	45.3
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.3	25.5
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	104.0	96.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,249.9	1,231.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	762.4	750.5
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	261.2	274.4
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.3	72.5
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	57.2	31.6
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	13.3	21.8
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	24.3	25.5
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	54.2	55.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	444.8	435.1
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.9	27.7
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	258.6	273.6
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.3	36.8
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	53.2	32.2
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	13.0	23.5
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 49.8	$ 41.3