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Related Parties (Net Intercompany Payables) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
IPL [Member]
   
Net Intercompany Payables $ 82.0 $ 79.0
WPL [Member]
   
Net Intercompany Payables $ 48.0 $ 42.0