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Related Parties (Net Intercompany Payables) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
IPL [Member]
   
Net payables to Corporate Services $ 72.0 $ 82.0
WPL [Member]
   
Net payables to Corporate Services $ 47.0 $ 48.0