Derivative Instruments - Realized and Unrealized Gains and Losses on Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net			$ (11,465)	$ 68,420
Gain (loss) on natural gas derivatives, net	$ (12,685)	$ 28,022	(2,511)	74,880
Diesel Fuel [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	185	0	919
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	(1,098)	0	(3,729)
Gain (loss) on natural gas derivatives, net	0	(913)	0	(2,810)
Swap [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	4,758	6,709	8,954	12,187
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	(17,443)	12,520	(11,465)	35,416
Collars | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	(3,050)	0	(9,456)
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	11,843	0	36,733
Crude Oil and Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	4,758	3,659	8,954	2,731
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	(17,443)	24,363	(11,465)	72,149
Gain (loss) on natural gas derivatives, net	$ (12,685)	$ 28,022	$ (2,511)	$ 74,880