Supplemental Oil and Gas Reserve Information - Unaudited - Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Standardized Measure of Discounted Future Net Cash Flows
January 1	$ 1,914,330	$ 3,981,661	$ 3,727,559
Net changes in prices and production costs	(367,527)	(3,201,195)	(588,212)
Net changes in future development costs	69,992	150,333	(61,760)
Sales of oil and natural gas, net	(403,739)	(477,755)	(979,938)
Purchases of reserves in place	533,505	14,378	38,035
Sales of reserves in place	(57,770)	0	(251,002)
Revisions of previous quantity estimates	333,398	(946,729)	(604,651)
Previously estimated development costs incurred	91,518	216,981	249,926
Accretion of discount	(36,303)	548,141	548,690
Net change in income taxes	202,272	1,391,358	259,592
Changes in timing and other	37,463	(172,681)	(107,585)
December 31	2,483,065	1,914,330	3,981,661
Extensions
Standardized Measure of Discounted Future Net Cash Flows
Proved oil and gas reserves increase	165,926	409,838	1,751,007
Discoveries
Standardized Measure of Discounted Future Net Cash Flows
Proved oil and gas reserves increase	$ 0	$ 0	$ 0