Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 60.5	$ 26.2
Derivative assets, Net	46.3	19.3
Derivative liabilities, Gross (as reported)	37.1	40.4
Derivative liabilities, Net	22.9	33.0
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	45.1	17.5
Derivative assets, Net	43.4	14.5
Derivative liabilities, Gross (as reported)	9.7	16.1
Derivative liabilities, Net	8.0	12.6
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	15.4	8.7
Derivative assets, Net	2.9	4.8
Derivative liabilities, Gross (as reported)	27.4	24.3
Derivative liabilities, Net	$ 14.9	$ 20.4