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Derivative Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Derivative fair value      
Maximum period covering risk of changes in supply of gas prices (in months) 18    
Open derivative contracts for natural gas (in MMBtus) 112,400,000   156,300,000
Percentage of natural gas consumption covered by derivatives 79.00%    
Aggregate fair value of the derivative instruments with credit risk related contingent features in a net liability position $ 130.3   $ 74.7
Natural gas derivatives not designated as hedging instruments
     
Derivative fair value      
Realized gains (losses) (48.2) (9.6)  
Unrealized mark-to-market (losses) gains (55.9) 0.7  
Net derivatives losses (104.1) (8.9)  
Unrealized gains in other current assets 0.2   0.5
Unrealized losses in other current liabilities (130.2)   (74.7)
Unrealized losses in noncurrent liabilities (0.1)    
Net unrealized derivative gains (losses) $ (130.1)   $ (74.2)