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Related Parties (Net Intercompany Payables) (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
IPL [Member]
   
Related Party Transaction [Line Items]    
Net payables to Corporate Services $ 73 $ 72
WPL [Member]
   
Related Party Transaction [Line Items]    
Net payables to Corporate Services $ 39 $ 40