Fair Value Measurements - Property Impairments (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value Disclosures [Abstract]
Proved property impairments	$ 39,635,000	$ 4,332,000	$ 39,635,000	$ 4,332,000
Unproved property impairments	40,077,000	31,539,000	80,158,000	61,446,000
Total	79,712,000	35,871,000	119,793,000	65,778,000
Oil and gas property fair value after impairment	$ 22,200,000	$ 2,200,000	$ 22,200,000	$ 2,200,000