Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets	$ 1,417.3	$ 1,413.2
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	834.0	829.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	351.8	355.3
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	68.5	65.7
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.8	25.0
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.2	30.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	9.1	21.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	96.9	86.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,118.0	1,113.5
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	808.3	798.6
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	172.6	174.2
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	38.6	36.7
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.0	21.0
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.2	30.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	2.5	5.9
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.8	47.1
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	299.3	299.7
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.7	31.1
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	179.2	181.1
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.9	29.0
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.8	4.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.6	15.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 53.1	$ 39.3