Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 1,917.2	$ 1,908.6
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,002.7	987.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	570.0	579.5
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	96.1	92.4
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	86.6	70.6
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.2	45.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.7	26.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.3	35.9
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	65.6	70.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,405.9	1,402.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	972.0	958.2
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	293.6	298.1
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	54.1	50.8
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.5	28.2
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.7	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	33.7	37.6
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	511.3	506.0
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.7	29.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	276.4	281.4
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.0	41.6
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	61.1	42.4
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.2	45.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.0	33.1
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 31.9	$ 33.0