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Fair Value Measurements - Summary of Changes of Derivative Commodity Instruments in Fair Value (Details) - Derivative Commodity Instruments - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at beginning of period $ (16,059) $ 24,436
Realized gains (losses) (5,518) 13,153
Unrealized gains (losses) relating to instruments held at the reporting date 7,905 (22,023)
Settlements during period 5,518 (13,153)
Balance at end of period (8,154) 2,413
Mark-to-market gain (loss) included in earnings $ 2,200 $ (10,300)