Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets	$ 1,453.2	$ 1,413.2
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	870.7	829.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	348.4	355.3
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	71.6	65.7
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.7	30.0
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.2	25.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	12.5	21.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	94.1	86.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,152.6	1,113.5
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	844.1	798.6
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	171.2	174.2
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	40.9	36.7
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.7	30.0
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.5	21.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.1	5.9
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	39.1	47.1
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	300.6	299.7
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.6	31.1
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	177.2	181.1
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.7	29.0
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.7	4.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	6.4	15.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 55.0	$ 39.3