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Fair Value Measurements - Significant Unobservable Inputs (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Significant Unobservable Inputs [Line Items]    
Total net derivative contracts $ (2,442) $ 19,992
Fair Value Measurements Level 3 | Oil Basis Swaps
    
Significant Unobservable Inputs [Line Items]    
Derivative, weighted average forward price (in dollars per Bbl) 13.28 14.74
Total net derivative contracts $ (211) $ (512)
Fair Value Measurements Level 3 | Oil Basis Swaps | Minimum
    
Significant Unobservable Inputs [Line Items]    
Derivative, forward price (in dollars per Bbl) 11.31 10.00
Fair Value Measurements Level 3 | Oil Basis Swaps | Maximum
    
Significant Unobservable Inputs [Line Items]    
Derivative, forward price (in dollars per Bbl) 15.49 21.98