Fair Value Measurements - Property Impairments (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value Disclosures [Abstract]
Proved property impairments	$ 0	$ 0	$ 39,635,000	$ 4,332,000
Unproved property impairments	42,167,000	27,375,000	122,325,000	88,821,000
Total	42,167,000	27,375,000	161,960,000	93,153,000
Oil and gas property fair value after impairment	$ 22,200,000	$ 2,200,000	$ 22,200,000	$ 2,200,000