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Derivatives - Cash Settlements and Valuation Gains and Losses on Derivative Contracts and Interest Rate Swaps (Parenthetical) (Details) (Commodity Derivatives, USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Derivative Instruments, (Gain) Loss      
Realized (gain) loss $ (31,718,000) [1] $ 50,713,000 [1] $ (224,337,000) [1]
Derivative Contracts Early Settlements
      
Derivative Instruments, (Gain) Loss      
Realized (gain) loss (59,500,000) (48,100,000) (114,500,000)
Realized (gain) on derivatives   (111,000,000)  
Realized loss on derivatives   62,900,000  
Amended
      
Derivative Instruments, (Gain) Loss      
Realized loss on derivatives $ 117,108,000    
[1] The year ended December 31, 2012 includes $59.5 million of net realized gain related to settlements of commodity derivative contracts with contractual maturities after the quarterly period in which they were settled (“early settlements”) and a $117.1 million non-cash realized loss on derivative contracts amended in January 2012. The years ended December 31, 2011 and 2010 include $48.1 million ($111.0 million realized gain and $62.9 million realized loss) and $114.5 million of realized gain, respectively, related to early settlements.