Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Oil and gas properties:
Proved	$ 2,682,932	$ 2,678,415
Unproved	6,037	6,881
Total oil and gas properties	2,688,969	2,685,296
Other property and equipment	31,563	31,365
Total properties and equipment	2,720,532	2,716,661
Accumulated depreciation, depletion and amortization	(2,386,026)	(2,372,266)
Property and equipment, net (successful efforts method)	$ 334,506	$ 344,395