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Receivables (Advances For Customer Energy Efficiency Projects) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Customer advances, current portion $ 8.2 $ 14.9
Customer advances, non-current portion 7.3 13.0
Customer advances 15.5 27.9
IPL [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Customer advances, current portion 0.4 0.8
Customer advances, non-current portion 0.2 0.6
Customer advances 0.6 1.4
WPL [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Customer advances, current portion 7.8 14.1
Customer advances, non-current portion 7.1 12.4
Customer advances $ 14.9 $ 26.5