Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 1,666.7	$ 1,915.1
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	778.2	1,055.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	548.0	578.7
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	109.3	105.9
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	63.8	41.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.3	30.7
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	26.2
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	96.6	76.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,231.6	1,458.8
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	750.5	1,022.4
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	274.4	294.0
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	72.5	64.3
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.6	0.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.8	10.0
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	26.2
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.3	41.9
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	435.1	456.3
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.7	33.2
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	273.6	284.7
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.8	41.6
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.2	41.4
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.5	20.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 41.3	$ 34.7