Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 1,915.1	$ 1,908.6
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,055.6	987.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	578.7	579.5
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	105.9	92.4
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.4	45.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.7	70.6
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.2	26.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.0	35.9
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	70.6	70.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,458.8	1,402.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,022.4	958.2
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	294.0	298.1
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	64.3	50.8
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.0	28.2
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.2	26.9
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0.3	2.8
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.6	37.6
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	456.3	506.0
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.2	29.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	284.7	281.4
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.6	41.6
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.4	45.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.7	42.4
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.7	33.1
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 29.0	$ 33.0