Capitalized Costs Related to Oil and Natural Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Oil and natural gas properties
Proved	$ 8,969,296		$ 8,159,924		$ 5,913,408
Unproved	689,393		547,953		281,811
Total oil and natural gas properties	9,658,689		8,707,877		6,195,219
Less accumulated depreciation, depletion and impairment	(4,791,534)	[1]	(4,483,736)	[1]	(4,223,437)
Net oil and natural gas properties capitalized costs	$ 4,867,155		$ 4,224,141		$ 1,971,782

[1] Includes cumulative full cost ceiling limitation impairment charges of $3,548.3 million at both December 31, 2011 and 2010. See Note 8 for further discussion of impairment charges. There were no full cost ceiling impairments during the years ended December 31, 2011 and 2010.