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
Supplemental Information on Oil and Gas Producing Activities [Abstract]
Sales of oil and gas, net of production costs	$ (254,388)	$ (234,211)	$ (180,568)
Net changes in prices and production costs	(207,045)	(25,398)	180,316
Extensions, discoveries and other additions	355,495	361,284	59,729
Development costs incurred during the period	119,706	44,741	153,563
Revisions of previous quantity estimates	(196,152)	(113,188)	(50,471)
Purchases of reserves-in-place	1,156	308	2,239
Sale of reserves-in-place	(116,151)	(37,474)	(47,740)
Accretion of discount	87,441	87,815	68,817
Net change in income taxes	25,312	16,818	(73,332)
Other changes	28,004	(87,618)	4,095
Net increase (decrease)	(156,622)	13,077	116,648
Beginning of year	654,496	641,419	524,771
End of year	$ 497,874	$ 654,496	$ 641,419