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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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance at beginning of period $ 24,502 $ 5,574 $ (29,288) $ (8,852)
Realized gains 4,518 417 (3,322) (2,845)
Unrealized gains relating to instruments held at the reporting date 68,138 1,876 121,929 16,301
Settlements during period (6,855) (222) 984 3,041
Balance at end of period 90,303 7,645 90,303 7,645
Mark-to-market gain (loss) included in earnings $ 68,300 $ 3,100 $ 104,400 $ 11,000