Derivative Instruments - Realized and Unrealized Gains and Losses on Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	$ 58,031	$ (156,621)	$ 21,532
Gain (loss) on crude oil and natural gas derivatives, net	$ 8,165		$ 8,602		$ 28,022		$ 46,858		$ (47,382)		$ 15,668		$ (82,257)		$ 42,112		91,647	(71,859)	91,085
Fuel [Member]
Cash received (paid) on derivatives:
Cash received 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 on derivatives, net																	40,095	88,823	39,670
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	18,960	(120,784)	41,828
Collars [Member] | Natural Gas [Member]
Cash received (paid) on derivatives:
Cash received on derivatives, net																	(10,539)	0	29,883
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	43,131	(39,936)	(25,011)
Call Option [Member] | Crude Oil [Member]
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	0	38	4,715
Crude Oil and Natural Gas
Cash received (paid) on derivatives:
Cash received on derivatives, net																	29,556	88,823	69,553
Non-cash gain (loss) on derivatives:
Non-cash gain (loss) on derivatives, net																	62,091	(160,682)	21,532
Gain (loss) on crude oil and natural gas derivatives, net																	$ 91,647	$ (71,859)	$ 91,085

[1] Gains and losses on crude oil and 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. Crude oil and 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.