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Fair Value Measurements - Reconciliation of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs (Details) (Commodity derivative contracts, USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Commodity derivative contracts
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Beginning balance $ (211) $ (512)
Gain (loss) on derivative contracts 740 (133)
Settlements (received) paid (529) 645
Ending balance $ 0 $ 0