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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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance at beginning of period $ 90,303 $ 7,645 $ (29,288) $ (8,852)
Realized gains (losses) 22,603 (1,548) 21,618 (4,588)
Unrealized gains (losses) relating to instruments held at the reporting date (109,528) (24,112) 10,064 (7,616)
Settlements during period (26,884) 398 (25,900) 3,439
Balance at end of period (23,506) (17,617) (23,506) (17,617)
Mark-to-market gain (loss) included in earnings $ 88,800 $ 23,000 $ 13,000 $ 14,200