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, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Standardized measure of discounted future net cash flows at beginning of year	$ 7,505,356	$ 3,785,322	$ 1,841,540
Extensions, discoveries and improved recoveries, less related costs	3,724,136	2,276,355	926,938
Revisions of previous quantity estimates	254,493	133,990	490,563
Changes in estimated future development and abandonment costs	(298,148)	(70,219)	(376,848)
Purchases (sales) of minerals in place	1,171,047	56,246	8,022
Net change in prices and production costs	(530,515)	1,855,532	1,177,446
Accretion of discount	750,536	378,532	184,154
Sales of crude oil and natural gas produced, net of production costs	(1,955,555)	(1,364,373)	(778,662)
Development costs incurred during the period	1,095,156	528,737	356,992
Change in timing of estimated future production and other	(102,519)	773,279	397,669
Change in income taxes	(433,630)	(848,045)	(442,492)
Net change	3,675,001	3,720,034	1,943,782
Standardized measure of discounted future net cash flows at end of year	$ 11,180,357	$ 7,505,356	$ 3,785,322