Summary of Property and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Oil and gas properties:
Proved	$ 2,970,047	$ 2,277,811
Unproved	101,520	60,746
Total oil and gas properties	3,071,567	2,338,557
Other property and equipment	105,421	93,648
Total property and equipment	3,176,988	2,432,205
Accumulated depreciation, depletion and amortization	(939,684)	(708,846)
Property and equipment, net (successful efforts method)	$ 2,237,304	$ 1,723,359