Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforward	$ 173,503	$ 176,812
Deferred compensation	45,413	73,942
Equity compensation	25,940	25,833
AMT credits and other credits	4,437	4,447
Asset retirement obligation	101,142	109,808
Cumulative mark-to-market loss	0	649
Other	10,163	10,908
Valuation allowances:
Total deferred tax assets	272,975	385,800
Deferred tax liabilities:
Depreciation, depletion and investments	(940,482)	(1,342,038)
Cumulative mark-to-market gain	(109,845)	(154,183)
Other	(595)	(2,660)
Total deferred tax liabilities	(1,050,922)	(1,498,881)
Net deferred tax liability	(777,947)	(1,113,081)
Federal
Valuation allowances:
Valuation allowances	(42,500)	0
State, net of federal benefit
Valuation allowances:
Valuation allowances	(41,516)	(8,800)
Deferred compensation plans and other
Valuation allowances:
Valuation allowances	$ (3,607)	$ (7,799)