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Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) (Commodity derivatives [Member], USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Commodity derivatives [Member]
    
Offsetting Liabilities [Line Items]    
Gross Amounts of Recognized Liabilities $ 46.9 $ 45.5
Gross Amounts Offset in the Balance Sheet 0 0
Amounts of Liabilities Presented in the Balance Sheet 46.9 45.5
Financial Instruments (43.2) (38.7)
Cash Collateral Paid 0 (4.3)
Amounts That Would Have Been Presented On Net Basis $ 3.7 $ 2.5