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Fair Value Measurements - Derivative Instruments Measured at Fair Value (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Balance, at beginning of period $ (25,134) $ (25,867) $ (21,254)
Settlements 0 16,810 7,691
Acquired 0 0 (2,844)
Total unrealized income (loss) included in earnings (1,238) (16,077) (9,460)
Balance, at end of period $ (26,372) $ (25,134) $ (25,867)