Summary of Property and Equipment (Detail) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Oil and gas properties:
Proved	$ 2,248,553	$ 2,139,894
Unproved, net of amortization	162,962	171,303
Total oil and gas properties	2,411,515	2,311,197
Other property and equipment	16,751	15,589
Total property and equipment	2,428,266	2,326,786
Accumulated depreciation, depletion and amortization	(676,005)	(621,202)
Property and equipment, net (successful efforts method)	$ 1,752,261	$ 1,705,584