Fair Value Measurements - Property Impairments (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value Disclosures [Abstract]
Proved property impairments	$ 38,046,000	$ 0	$ 69,337,000	$ 39,635,000
Unproved property impairments	47,515,000	42,167,000	153,748,000	122,325,000
Total	85,561,000	42,167,000	223,085,000	161,960,000
Oil and gas property fair value after impairment	$ 15,400,000	$ 22,200,000	$ 15,400,000	$ 22,200,000