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Related Parties (Service Agreements) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Corporate Service Billings [Member] | IPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties $ 33 $ 32
Corporate Service Billings [Member] | WPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties 28 26
Sales Credited [Member] | IPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties 2 2
Sales Credited [Member] | WPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties (1) 3
Purchases Billed [Member] | IPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties 99 76
Purchases Billed [Member] | WPL [Member]
   
Related Party Transaction [Line Items]    
Amounts billed between related parties $ 30 $ 14