Changes in Standardized Measure of Discounted Future Net Cash Flows (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]
Sales of oil and gas, net of production costs	$ (234,211)	$ (180,568)	$ (157,891)
Net changes in prices and production costs	(25,398)	180,316	(314,209)
Extensions, discoveries and other additions	361,284	59,729	138,482
Development costs incurred during the period	44,741	153,563	65,043
Revisions of previous quantity estimates	(113,188)	(50,471)	(158,844)
Purchases of reserves-in-place	308	2,239
Sale of reserves-in-place	(37,474)	(47,740)
Accretion of discount	87,815	68,817	90,796
Net change in income taxes	16,818	(73,332)	15,168
Other changes	(87,618)	4,095	116,825
Net increase (decrease)	13,077	116,648	(204,630)
Beginning of year	641,419	524,771	729,401
End of year	$ 654,496	$ 641,419	$ 524,771