Summary of Accounts Receivable (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customers	$ 42,079	$ 49,763
Joint interest partners	25,485	22,755
Other	3,738	1,695
Accounts Receivable, Gross, Current, Total	71,302	74,213
Less: Allowance for doubtful accounts	(1,263)	(1,781)
Accounts receivable, net of allowance for doubtful accounts	$ 70,039	$ 72,432