Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 23.6	$ 26.2
Derivative assets, Net	16.3	19.3
Derivative liabilities, Gross (as reported)	17.1	40.4
Derivative liabilities, Net	9.8	33.0
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	12.5	17.5
Derivative assets, Net	9.0	14.5
Derivative liabilities, Gross (as reported)	6.1	16.1
Derivative liabilities, Net	2.6	12.6
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	11.1	8.7
Derivative assets, Net	7.3	4.8
Derivative liabilities, Gross (as reported)	11.0	24.3
Derivative liabilities, Net	$ 7.2	$ 20.4