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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
Mar. 31, 2016
Mar. 31, 2015
Level 3 Fair Value Measurements - Commodity Derivative Contracts    
Beginning balance $ (1,748) $ 350
Purchases 0 347
Loss on commodity derivative contracts 586 635
Ending balance $ (1,162) $ 1,332