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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 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Level 3 Fair Value Measurements - Commodity Derivative Contracts        
Beginning balance $ (356) $ (2,207) $ (1,748) $ 350
Purchases 0 0 0 (2,894)
Gain (loss) on commodity derivative contracts 186 (1,229) 1,578 (892)
Ending balance $ (170) $ (3,436) $ (170) $ (3,436)