Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforward	$ 542,847	$ 413,672
Deferred compensation	19,844	24,704
Equity compensation	8,152	5,269
AMT credits and other credits	3,296	7,264
Asset retirement obligation	78,126	69,398
Other	30,492	18,806
Valuation allowances:
Total deferred tax assets	562,410	413,979
Deferred tax liabilities:
Depreciation, depletion and investments	(1,207,784)	(1,105,494)
Cumulative mark-to-market gain	(21,294)	(1,841)
Other	0	0
Total deferred tax liabilities	(1,229,078)	(1,107,335)
Net deferred tax liability	(666,668)	(693,356)
Federal
Valuation allowances:
Valuation allowances	(18,975)	(31,308)
State, net of federal benefit
Valuation allowances:
Valuation allowances	$ (101,372)	$ (93,826)