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Fair Value Measurements - Additional Information (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Total net derivative contracts $ 19,992 $ (134,649)  
Unrealized gain (loss) on derivatives 217,755 101,034 (283,604)
Fair Value Measurements Level 3
     
Unrealized gain (loss) on derivatives (500)    
Fair Value Measurements Level 3 | Oil basis swaps
     
Total net derivative contracts $ (512)    
Derivative, weighted average forward price (in dollars per barrel) 14.74    
Fair Value Measurements Level 3 | Oil basis swaps | Minimum
     
Derivative, forward price (in dollars per barrel) 10.00    
Fair Value Measurements Level 3 | Oil basis swaps | Maximum
     
Derivative, forward price (in dollars per barrel) 21.98