Derivative Instruments - Realized and Unrealized Gains and Losses on Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	$ 5,978	$ 45,155
Gain on natural gas derivatives, net	10,174	46,858
Diesel Fuel [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	734
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	(2,631)
Gain on natural gas derivatives, net	0	(1,897)
Swap [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	4,196	5,478
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	5,978	22,896
Collars | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	0	(6,406)
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	0	24,890
Crude Oil and Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net	4,196	(928)
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net	5,978	47,786
Gain on natural gas derivatives, net	$ 10,174	$ 46,858