Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 1,874.8	$ 1,908.6
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,033.5	987.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	551.0	579.5
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	103.9	92.4
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.1	45.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.0	70.6
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.3	26.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.1	35.9
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	67.9	70.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,417.2	1,402.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,000.7	958.2
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	284.7	298.1
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	61.5	50.8
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.6	28.2
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.3	26.9
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0.4	2.8
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.0	37.6
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	457.6	506.0
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.8	29.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	266.3	281.4
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.4	41.6
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.1	45.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.4	42.4
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	9.7	33.1
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 34.9	$ 33.0