Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,664.1	$ 1,666.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	795.1	778.2
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	538.6	548.0
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	104.3	109.3
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	60.1	63.8
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	47.6	45.3
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.4	25.5
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	93.0	96.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,230.9	1,231.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	762.8	750.5
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	270.0	274.4
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	67.9	72.5
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.4	31.6
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	21.8
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.4	25.5
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	48.9	55.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	433.2	435.1
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.3	27.7
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	268.6	273.6
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.4	36.8
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.7	32.2
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.1	23.5
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 44.1	$ 41.3