Supplemental Crude Oil and Natural Gas Information - Changes in Standardized Measure of Discounted Future Net Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure of discounted future net cash flows at beginning of year	$ 5,510,223	$ 6,476,284	$ 18,433,034
Extensions, discoveries and improved recoveries, less related costs	1,462,629	786,587	1,091,283
Revisions of previous quantity estimates	(1,004,355)	(794,785)	(2,156,028)
Changes in estimated future development and abandonment costs	743,657	1,651,218	5,008,731
Sales of Minerals in Place	(41,077)	(90,390)	(7,768)
Net change in prices and production costs	3,808,116	(2,003,163)	(16,111,142)
Accretion of discount	665,507	798,597	1,843,303
Sales of crude oil and natural gas produced, net of production costs	(2,450,474)	(1,595,281)	(2,002,997)
Development costs incurred during the period	1,045,875	454,983	1,394,584
Change in timing of estimated future production and other	948,519	(538,665)	(3,844,259)
Change in income taxes	(218,443)	364,838	2,827,543
Net change	4,959,954	(966,061)	(11,956,750)
Standardized measure of discounted future net cash flows at end of year	$ 10,470,177	$ 5,510,223	$ 6,476,284