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Fair Value Measurements - Reconciliation of Fair Value Measurements for Commodity Derivative Contracts (Details) - Commodity derivative contracts - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Level 3 Fair Value Measurements - Commodity Derivative Contracts        
Beginning balance $ (1,162) $ 1,332 $ (1,748) $ 350
Purchases 0 0 0 (1,847)
Gain (loss) on commodity derivative contracts 806 (3,539) 1,392 (710)
Ending balance $ (356) $ (2,207) $ (356) $ (2,207)