Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 1,790.3	$ 1,783.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	968.2	955.3
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	562.9	570.2
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	75.6	73.7
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	49.6	46.9
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.1	31.1
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.9	27.4
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	75.0	79.1
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,363.7	1,357.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	940.5	928.0
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	284.8	287.9
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.5	41.4
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.4	28.0
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0.6	0.4
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.9	27.4
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	42.0	44.8
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	426.6	425.8
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.7	27.3
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	278.1	282.3
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.1	32.3
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.2	18.9
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.5	30.7
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 33.0	$ 34.3