Derivative Instruments - Realized and Unrealized Gains and Losses on Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net			$ (12,013)	$ 65,481
Gain (loss) on natural gas derivatives, net	$ (2,025)	$ 8,602	(4,536)	83,482
Diesel Fuel [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	603	0	1,522
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	740	0	(2,989)
Gain (loss) on natural gas derivatives, net	0	1,343	0	(1,467)
Swap [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	(1,477)	12,893	7,477	25,080
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	(548)	(8,026)	(12,013)	27,390
Collars | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	(612)	0	(10,068)
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	4,347	0	41,080
Crude Oil and Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	(1,477)	12,281	7,477	15,012
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	(548)	(3,679)	(12,013)	68,470
Gain (loss) on natural gas derivatives, net	$ (2,025)	$ 8,602	$ (4,536)	$ 83,482