Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 36.8	$ 26.2
Derivative assets, Net	32.3	19.3
Derivative liabilities, Gross (as reported)	27.1	40.4
Derivative liabilities, Net	23.5	33.0
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	31.5	17.5
Derivative assets, Net	29.2	14.5
Derivative liabilities, Gross (as reported)	6.7	16.1
Derivative liabilities, Net	5.3	12.6
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	5.3	8.7
Derivative assets, Net	3.1	4.8
Derivative liabilities, Gross (as reported)	20.4	24.3
Derivative liabilities, Net	$ 18.2	$ 20.4