Standardized Measure of Discounted Future Net Cash Flows Attributable to Proved Reserves (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 5,032,915	$ 4,833,030	$ 4,178,449
Future production costs	(1,374,658)	(1,388,857)	(1,300,235)
Future development costs	(1,091,100)	(879,193)	(888,493)
Future net cash flows before income tax	2,567,157	2,564,980	1,989,721
Future income tax expense	(665,751)	(687,928)	(491,832)
Future net cash flows	1,901,406	1,877,052	1,497,889
10% annual discount for estimated timing of cash flows	(1,246,910)	(1,235,633)	(973,118)
Standardized measure of discounted future net cash flows	$ 654,496	$ 641,419	$ 524,771