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, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of income taxes amount
U.S. statutory rate		$ (57,312)	$ 32,505	$ 43,820
State income taxes, net of federal tax benefit		(3,474)	1,882	2,097
Mining depletion		(1,557)	(3,035)	(2,751)
Change in election to deduct foreign taxes paid		1,442
Foreign tax assets valuation allowance	$ 4,300	1,230	4,300
Non-recognized benefit on foreign investments	$ 2,980	847	2,980
Section 199 Manufacturing Benefit and other		4,619	(1,349)	(2,851)
Income taxes		$ (54,205)	$ 37,283	$ 40,315
Reconciliation of income taxes rate
U.S. statutory rate		(35.00%)	35.00%	35.00%
State income taxes, net of federal tax benefit		(2.10%)	2.00%	1.70%
Mining depletion		(0.90%)	(3.30%)	(2.20%)
Change in election to deduct foreign taxes paid		0.90%
Foreign tax assets valuation allowance		0.70%	4.60%
Non-recognized benefit on foreign investments		0.50%	3.20%
Section 199 Manufacturing Benefit and other		2.80%	(1.40%)	(2.30%)
Effective Income Tax Rate, Continuing Operations, Total		(33.10%)	40.10%	32.20%