Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 1,872.6	$ 1,908.6
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,020.1	987.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	560.5	579.5
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	99.7	92.4
WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.5	45.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.7	70.6
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.6	26.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.8	35.9
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	65.7	70.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,405.4	1,402.6
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	988.1	958.2
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	289.2	298.1
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	57.7	50.8
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	11.0	28.2
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.6	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	32.5	37.6
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	467.2	506.0
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.0	29.5
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	271.3	281.4
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.0	41.6
WPL [Member] | WPL's EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.5	45.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.7	42.4
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.5	33.1
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 33.2	$ 33.0