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Derivative Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Derivative fair value          
Maximum period covering risk of changes in supply of gas prices     18 months    
Open derivative contracts for natural gas (in MMBtus) 38,700,000   38,700,000   156,300,000
Percentage of natural gas consumption covered by derivatives 67.00%   67.00%    
Aggregate fair value of the derivative instruments with credit risk related contingent features in a net liability position $ 12.7   $ 12.7   $ 74.7
Natural gas derivatives not designated as hedging instruments
         
Derivative fair value          
Realized losses (31.7) (15.8) (134.1) (24.5)  
Unrealized mark-to-market gains (losses) 39.8 (14.1) 61.5 (27.6)  
Net derivative gains (losses) 8.1 (29.9) (72.6) (52.1)  
Unrealized gains in other current assets 0.9   0.9   0.5
Unrealized losses in other current liabilities (13.6)   (13.6)   (74.7)
Net unrealized derivative losses $ (12.7)   $ (12.7)   $ (74.2)