Supplemental Quarterly Financial Information (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended			3 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2010 Assets Held-for-sale
Quarterly Financial Information [Line Items]
Impairement of oil and gas properties	$ 34,000	$ 71,000	$ 9,700	$ 35,100	$ 104,688	$ 43,067	$ 102,332	$ 1,100