Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 1,807.9	$ 1,783.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	976.5	955.3
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	548.4	570.2
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	81.8	73.7
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	68.4	46.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.2	31.1
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.5	27.4
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	73.1	79.1
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,370.2	1,357.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	947.3	928.0
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	278.6	287.9
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	47.6	41.4
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.3	28.0
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	1.2	0.4
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.5	27.4
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.7	44.8
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	437.7	425.8
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.2	27.3
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	269.8	282.3
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	34.2	32.3
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.1	18.9
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.0	30.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 33.4	$ 34.3