Supplemental Crude Oil and Natural Gas Information - Changes in Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	$ 16,295,767	$ 11,180,357	$ 7,505,356
Extensions, discoveries and improved recoveries, less related costs	5,516,528	6,613,665	3,724,136
Revisions of previous quantity estimates	(1,755,366)	(1,765,300)	254,493
Changes in estimated future development and abandonment costs	476,665	1,942,585	(298,148)
Decrease Due to Sales of Minerals in Place	(3,196)
Purchases (sales) of minerals in place		12,012	1,171,047
Net change in prices and production costs	(1,925,349)	263,541	(530,515)
Accretion of discount	1,629,576	1,118,036	750,536
Sales of crude oil and natural gas produced, net of production costs	(3,500,790)	(2,992,447)	(1,955,555)
Development costs incurred during the period	2,466,748	1,210,223	1,095,156
Change in timing of estimated future production and other	(309,902)	464,111	(102,519)
Change in income taxes	(457,647)	(1,751,016)	(433,630)
Net change	2,137,267	5,115,410	3,675,001
Standardized measure of discounted future net cash flows at end of year	$ 18,433,034	$ 16,295,767	$ 11,180,357