Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforward	$ 478,203	$ 173,503
Deferred compensation	50,808	45,413
Equity compensation	29,528	25,940
AMT credits and other credits	13,644	4,437
Asset retirement obligation	99,000	101,142
Cumulative mark-to-market loss	73,404	0
Other	39,922	10,163
Valuation allowances:
Total deferred tax assets	677,335	272,975
Deferred tax liabilities:
Depreciation, depletion and investments	(1,619,922)	(940,482)
Cumulative mark-to-market gain	0	(109,845)
Other	(756)	(595)
Total deferred tax liabilities	(1,620,678)	(1,050,922)
Net deferred tax liability	(943,343)	(777,947)
Federal
Valuation allowances:
Valuation allowances	(43,600)	(42,500)
State, net of federal benefit
Valuation allowances:
Valuation allowances	(58,424)	(41,516)
Deferred compensation plans and other
Valuation allowances:
Valuation allowances	$ (5,150)	$ (3,607)