Reconciliation of Income Taxes Computed at U.S. Statutory Tax Rate to Income Tax Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of income taxes amount
U.S. statutory rate	$ (45,923,000)	$ (57,312,000)	$ 32,505,000
State income taxes, net of federal tax benefit	(3,283,000)	(3,474,000)	1,882,000
Mining depletion	(378,000)	(1,557,000)	(3,035,000)
Change in election for foreign tax credits	(2,753,000)	1,442,000
Foreign tax assets valuation allowance	0	1,230,000	4,300,000
Foreign investments	(323,000)	847,000	2,980,000
Stock compensation excess tax deficiency	789,000
Other permanent differences	790,000	4,619,000	(1,349,000)
Income taxes	$ (51,081,000)	$ (54,205,000)	$ 37,283,000
Reconciliation of income taxes rate
U.S. statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	2.50%	2.10%	2.00%
Mining depletion	0.30%	0.90%	(3.30%)
Change in election for foreign tax credits	2.10%	(0.90%)
Foreign tax assets valuation allowance		(0.70%)	4.60%
Foreign investments	0.20%	(0.50%)	3.20%
Stock compensation excess tax deficiency	(0.60%)
Other permanent differences	(0.60%)	(2.80%)	(1.40%)
Effective Income Tax Rate, Continuing Operations, Total	38.90%	33.10%	40.10%