Summary of Property and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Oil and gas properties:
Proved	$ 3,533,859	$ 3,390,482
Unproved	127,504	125,676
Total oil and gas properties	3,661,363	3,516,158
Other property and equipment	76,058us-gaap_PropertyPlantAndEquipmentOther	75,073us-gaap_PropertyPlantAndEquipmentOther
Total properties and equipment	3,737,421us-gaap_PropertyPlantAndEquipmentGross	3,591,231us-gaap_PropertyPlantAndEquipmentGross
Accumulated depreciation, depletion and amortization	(1,856,809)	(1,766,133)
Property and equipment, net (successful efforts method)	$ 1,880,612us-gaap_PropertyPlantAndEquipmentNet	$ 1,825,098us-gaap_PropertyPlantAndEquipmentNet

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Oil and Gas properties, gross, carried under the successful effort method.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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