Summary of Accounts Receivable (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customers	$ 35,603	$ 49,763
Joint interest partners	18,516	22,755
Other	2,895	1,695
Accounts Receivable, Gross, Current, Total	57,014	74,213
Less: Allowance for doubtful accounts	(1,263)	(1,781)
Accounts receivable, net of allowance for doubtful accounts	$ 55,751	$ 72,432