Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets	$ 1,783.7	$ 1,413.2
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	955.3	829.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	570.2	355.3
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	73.7	65.7
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.9	21.1
Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.1	2.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.4	30.0
Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.1	17.9
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	16.0	25.0
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	47.0	66.5
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,357.9	1,113.5
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	928.0	798.6
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	287.9	174.2
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.4	36.7
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.0	5.9
IPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0.4	0.7
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.4	30.0
IPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	10.9	12.2
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	11.5	21.0
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.4	34.2
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	425.8	299.7
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.3	31.1
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	282.3	181.1
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	32.3	29.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	18.9	15.2
WPL [Member] | Commodity cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.7	1.3
WPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.2	5.7
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.5	4.0
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 24.6	$ 32.3