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Fair Value Measurements - Significant Unobservable Inputs (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Significant Unobservable Inputs    
Total net derivative contracts $ (37,229) $ 19,992
Fair Value Measurements Level 3 | Oil Basis Swaps
    
Significant Unobservable Inputs    
Derivative, weighted average forward price (in dollars per Bbl)   14.74
Total net derivative contracts $ 0 $ (512)
Fair Value Measurements Level 3 | Oil Basis Swaps | Minimum
    
Significant Unobservable Inputs    
Derivative, forward price (in dollars per Bbl)   10.00
Fair Value Measurements Level 3 | Oil Basis Swaps | Maximum
    
Significant Unobservable Inputs    
Derivative, forward price (in dollars per Bbl)   21.98