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Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Compensation expense $ 10.1 $ 7.5 $ 2.8
Income tax benefits 4.0 3.0 1.2
IPL [Member]
     
Compensation expense 5.5 4.0 1.6
Income tax benefits 2.2 1.6 0.7
WPL [Member]
     
Compensation expense 4.1 3.0 1.1
Income tax benefits $ 1.7 $ 1.2 $ 0.4