Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets, Gross (as reported)	$ 26.7	$ 26.2
Derivative assets, Net	23.5	19.3
Derivative liabilities, Gross (as reported)	20.8	40.4
Derivative liabilities, Net	17.6	33.5
IPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	21.1	17.5
Derivative assets, Net	19.5	14.5
Derivative liabilities, Gross (as reported)	5.2	16.1
Derivative liabilities, Net	3.6	13.1
WPL [Member]
Derivative [Line Items]
Derivative assets, Gross (as reported)	5.6	8.7
Derivative assets, Net	4.0	4.8
Derivative liabilities, Gross (as reported)	15.6	24.3
Derivative liabilities, Net	$ 14.0	$ 20.4