Financial Data Related to Oil and Gas Producing Activities (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Asset retirement costs included in development cost	$ 95,293	$ 96,386	$ 56,444