Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforward	$ 413,672	$ 478,203
Deferred compensation	24,704	50,808
Equity compensation	5,269	29,528
AMT credits and other credits	7,264	13,644
Asset retirement obligation	69,398	99,000
Cumulative mark-to-market loss	0	73,404
Other	18,806	39,922
Valuation allowances:
Total deferred tax assets	413,979	677,335
Deferred tax liabilities:
Depreciation, depletion and investments	(1,105,494)	(1,619,922)
Cumulative mark-to-market gain	(1,841)	0
Other	0	(756)
Total deferred tax liabilities	(1,107,335)	(1,620,678)
Net deferred tax liability	(693,356)	(943,343)
Federal
Valuation allowances:
Valuation allowances	(31,308)	(48,750)
State, net of federal benefit
Valuation allowances:
Valuation allowances	$ (93,826)	$ (58,424)