Supplemental Crude Oil and Natural Gas Information - 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]
Future cash inflows	$ 54,362,574	$ 35,042,916	$ 20,420,667
Future production costs	(13,103,469)	(7,495,552)	(4,931,251)
Future development and abandonment costs	(8,295,130)	(5,073,043)	(3,517,389)
Future income taxes	(8,500,766)	(5,956,615)	(2,890,644)
Future net cash flows	24,463,209	16,517,706	9,081,383
10% annual discount for estimated timing of cash flows	(13,282,852)	(9,012,350)	(5,296,061)
Standardized measure of discounted future net cash flows	$ 11,180,357	$ 7,505,356	$ 3,785,322