Reconciliation of Income Taxes Computed at U.S. Statutory Tax Rate to Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of income taxes amount
U.S. statutory rate	$ (89,300)	$ (45,923)	$ (57,312)
State income taxes, net of federal tax benefit	(5,684)	(3,283)	(3,474)
Mining depletion	(619)	(378)	(1,557)
Change in election for foreign tax credits	(667)	(2,753)	1,442
Foreign tax assets valuation allowance			1,230
Foreign investments	8,569	(323)	847
Stock compensation excess tax deficiency	876	789
Other permanent differences	1,806	790	4,619
Tax reform deferred rate change	28,163
Valuation allowance	54,829
Income taxes	$ (2,027)	$ (51,081)	$ (54,205)
Reconciliation of income taxes rate
U.S. statutory rate	(35.00%)	(35.00%)	(35.00%)
State income taxes, net of federal tax benefit	(2.20%)	(2.50%)	(2.10%)
Mining depletion	(0.20%)	(0.30%)	(0.90%)
Change in election for foreign tax credits	(0.30%)	(2.10%)	0.90%
Foreign tax assets valuation allowance			0.70%
Foreign investments	3.40%	(0.20%)	0.50%
Stock compensation excess tax deficiency	0.30%	0.60%
Other permanent differences	0.70%	0.60%	2.80%
Tax reform deferred rate change	11.00%
Valuation allowance	21.50%
Effective Income Tax Rate, Continuing Operations, Total	(0.80%)	(38.90%)	(33.10%)