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
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 6,295,455	$ 6,171,279	$ 3,803,004
Future production costs	(1,946,426)	(1,747,806)	(1,191,718)
Future development costs	(1,241,531)	(1,219,214)	(907,956)
Future income taxes	(799,007)	(852,364)	(330,651)
Future net cash flows	2,308,491	2,351,895	1,372,679
10% annual discount	(794,632)	(808,933)	(415,050)
Standardized measure of discounted future net cash flows	1,513,859	1,542,962	957,629
Changes in standardized measure of future net cash flows related to estimated proved oil and gas reserves
Standardized measure at beginning of year	1,542,962	957,629	614,987
Sales and transfers of oil, gas and NGLs produced, net of production costs	(697,741)	(670,347)	(487,418)
Changes in price, net of future production costs	(380,841)	502,324	485,272
Extensions and discoveries, net of future production and development costs	178,272	293,168	270,629
Changes in estimated future development costs, net of development costs incurred during the period	212,329	97,852	119,986
Revisions of quantity estimates	76,450	(27,854)	147,509
Accretion of discount	207,292	118,722	64,836
Net change in income taxes	22,947	(300,363)	(196,219)
Purchases of reserves in-place	276,389	567,286	21,264
Sales of reserves in-place	2,480	(36,278)	1,424
Changes in production rates due to timing and other	73,320	40,823	(84,641)
Net increase (decrease) in standardized measure	(29,103)	585,333	342,642
Standardized measure at end of year	$ 1,513,859	$ 1,542,962	$ 957,629