Derivative Instruments - Schedule of Net Gains and Loss on Derivative Contracts (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
(Loss) gain on derivative contracts	$ (36,704)	$ 131	$ (100,920)	$ 2,385
Net cash (paid) received upon settlement of derivative contracts	(13,551)	2,255	(27,462)	2,385
Net cash received upon settlement of derivative contracts prior to contractual maturity	$ 0	$ 2,255	$ 377	$ 2,255