Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 1,777.9	$ 1,783.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	961.1	955.3
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	555.7	570.2
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	77.9	73.7
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	54.1	46.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.3	31.1
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.7	27.4
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	70.1	79.1
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,350.7	1,357.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	932.8	928.0
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	281.8	287.9
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	44.1	41.4
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.1	28.0
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0.5	0.4
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.7	27.4
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.7	44.8
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	427.2	425.8
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.3	27.3
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	273.9	282.3
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.8	32.3
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.0	18.9
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.8	30.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 31.4	$ 34.3