Derivative Instruments - Realized and Unrealized Gains and Losses on Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	$ 13,009	$ 58,031	$ (156,621)
Gain (loss) on crude oil and natural gas derivatives, net	$ (19,394)		$ (2,025)		$ (12,685)		$ 10,174		$ 8,165		$ 8,602		$ 28,022		$ 46,858		(23,930)	91,647	(71,859)
Fuel [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net																		2,845	699
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																		(4,060)	4,060
Gain (loss) on crude oil and natural gas derivatives, net																		(1,215)	4,759
Fixed Price Swaps [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net																	(36,939)	40,095	88,823
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	7,527	18,960	(120,784)
Collars [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net																	0	(10,539)	0
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	5,482	43,131	(39,936)
Call Option [Member] | Crude Oil [Member]
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	0	0	38
Crude Oil and Natural Gas
Cash received (paid) on derivatives:
Cash received (paid) on derivatives, net																	(36,939)	29,556	88,823
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	13,009	62,091	(160,682)
Gain (loss) on crude oil and natural gas derivatives, net																	$ (23,930)	$ 91,647	$ (71,859)

[1] Gains and losses on natural gas derivative instruments are reflected in “Total revenues” on both the consolidated statements of comprehensive income (loss) and this table of unaudited quarterly financial data. Natural gas derivative gains and losses have been shown separately to illustrate the fluctuations in revenues that are attributable to the Company’s derivative instruments. Commodity price fluctuations each quarter can result in significant swings in mark-to-market gains and losses, which affects comparability between periods. Additionally, beginning in 2018 certain transportation expenses are no longer netted within "Total revenues" as a result of the Company's January 1, 2018 prospective adoption of ASU 2016-08, which affects comparability of 2017 and 2018 revenues. Transportation expenses totaled $49.3 million, $47.3 million, $46.0 million, and $49.0 million for the first, second, third, and fourth quarters of 2018, respectively.