Supplemental Oil and Gas Disclosures - Results of Operations for Oil and Natural Gas Producing Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Revenues	$ 1,034,892	$ 625,233	$ 721,672
Production costs	321,007	222,117	245,675
Depreciation, depletion and amortization	515,600	462,320	472,800
Exploration expenses	11,600	1,785	2,369
Rig termination	0	0	3,895
Impairment	6,887	2,252	46,109
Income tax expense (benefit)	67,244	(23,665)	(18,382)
Results of operations for oil and natural gas producing activities	$ 112,554	$ (39,576)	$ (30,794)

- Definition

Exploration expenses (including prospecting) related to oil and gas producing entities and would be included in operating expenses of that entity. Costs incurred in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: (i) Costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. (ii) Costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, and the maintenance of land and lease records. (iii) Dry hole contributions and bottom hole contributions. (iv) Costs of drilling and equipping exploratory wells. (v) Costs of drilling exploratory-type stratigraphic test wells.

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