Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Oil and gas properties:
Proved	$ 3,625,962	$ 3,390,482
Unproved	128,317	125,676
Total oil and gas properties	3,754,279	3,516,158
Other property and equipment	76,719	75,073
Total properties and equipment	3,830,998	3,591,231
Accumulated depreciation, depletion and amortization	(1,942,106)	(1,766,133)
Property and equipment, net (successful efforts method)	$ 1,888,892	$ 1,825,098