Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 1,963.0	$ 1,915.1
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,080.2	1,055.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	568.5	578.7
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	108.6	105.9
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.4	30.7
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.7	41.4
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.0	26.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	93.6	76.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,500.8	1,458.8
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,044.9	1,022.4
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	289.1	294.0
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	67.1	64.3
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.2	10.0
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.0	26.2
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.5	41.9
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	462.2	456.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.3	33.2
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	279.4	284.7
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.5	41.6
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.2	20.7
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.7	41.4
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 37.1	$ 34.7