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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, 2018
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at beginning of period $ (29,288) $ (8,852)
Realized losses (8,881) (3,253)
Unrealized gains relating to instruments held at the reporting date 54,832 14,416
Settlements during period 7,839 3,263
Balance at end of period 24,502 5,574
Mark-to-market gain (loss) included in earnings $ 42,900 $ 10,900