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, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues	$ 300,046	$ 251,336	$ 228,659
Production expenses	65,835	63,854	72,255
Exploration expenses	78,943	49,641	57,754
Depreciation and depletion expense	160,293	130,816	150,429
Impairment of oil and gas properties	104,688	45,959	106,415
Results of Operations, Income before Income Taxes, Total	(109,713)	(38,934)	(158,194)
Income tax expense (benefit)	(42,788)	(15,184)	(61,221)
Results of operations	$ (66,925)	$ (23,750)	$ (96,973)