Reconciliation of Income Taxes Computed at U.S. Statutory Tax Rate to Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of income taxes amount
U.S. statutory rate		$ (15,658)	$ (89,300)
State income taxes, net of federal tax benefit		(2,152)	(5,684)
Mining depletion		(163)	(619)
Change in election for foreign tax credits		32	(667)
Foreign investments		271	8,569
Stock compensation excess tax deficiency		348	876
Other permanent differences		663	1,806
Tax reform deferred rate change	$ 28,163		28,163
Valuation allowance		17,551	54,829
Income taxes		$ 892	$ (2,027)
Reconciliation of income taxes rate
U.S. statutory rate		(21.00%)	(35.00%)
State income taxes, net of federal tax benefit		(2.90%)	(2.20%)
Mining depletion		(0.20%)	(0.20%)
Change in election for foreign tax credits			(0.30%)
Foreign investments		0.40%	3.40%
Stock compensation excess tax deficiency		0.50%	0.30%
Other permanent differences		0.90%	0.70%
Tax reform deferred rate change			11.00%
Valuation allowance		23.50%	21.50%
Effective Income Tax Rate, Continuing Operations, Total		1.20%	(0.80%)