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Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) (Commodity derivatives [Member], USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Commodity derivatives [Member]
    
Offsetting Assets [Line Items]    
Gross Amounts of Recognized Assets $ 60.8 $ 61.6
Gross Amounts Offset in the Balance Sheet 0 0
Amounts of Assets Presented in the Balance Sheet 60.8 61.6
Financial Instruments (43.2) (38.7)
Cash Collateral Received 0 (15.2)
Cash Collateral Paid (14.3) 0
Amounts That Would Have Been Presented On Net Basis $ 3.3 $ 7.7