Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 2,036.5	$ 1,915.1
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,147.9	1,055.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	547.8	578.7
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	107.9	105.9
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	67.4	41.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	49.9	30.7
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.6	26.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	90.0	76.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,590.9	1,458.8
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,107.9	1,022.4
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	279.3	294.0
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	71.9	64.3
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.9	0.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.3	10.0
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.6	26.2
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	51.0	41.9
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	445.6	456.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	40.0	33.2
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	268.5	284.7
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.0	41.6
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.5	41.4
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.6	20.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 39.0	$ 34.7