Results Solely from Production and Sale of Oil and Gas and Non-Cash Charges for Property Impairments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Information on Oil and Gas Producing Activities [Abstract]
Revenues	$ 310,484	$ 300,046	$ 251,336
Production expenses	56,096	65,835	63,854
Exploration expenses	34,092	78,943	49,641
Depreciation and depletion expense	204,849	160,293	130,816
Impairment of oil and gas properties	104,484	104,688	45,959
Results of Operations, Income before Income Taxes, Total	(89,037)	(109,713)	(38,934)
Income tax expense (benefit)	(34,724)	(42,788)	(15,184)
Results of operations	$ (54,313)	$ (66,925)	$ (23,750)