Reconciliation of Income Taxes Computed at U.S. Statutory Tax Rate to Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of income taxes amount
U.S. statutory rate		$ 32,505	$ 43,820	$ 55,507
State income taxes, net of federal tax benefit		1,882	2,097	2,199
Mining depletion		(3,035)	(2,751)	(2,606)
Foreign tax assets valuation allowance	4,300	4,300
Non-recognized benefit on foreign investments		2,980
Section 199 Manufacturing Benefit and other		(1,349)	(2,851)	(2,443)
Income taxes		$ 37,283	$ 40,315	$ 52,657
Reconciliation of income taxes rate
U.S. statutory rate		35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit		2.00%	1.70%	1.40%
Mining depletion		(3.30%)	(2.20%)	(1.60%)
Foreign tax assets valuation allowance		4.60%
Non-recognized benefit on foreign investments		3.20%
Section 199 Manufacturing Benefit and other		(1.40%)	(2.30%)	(1.60%)
Effective Income Tax Rate, Continuing Operations, Total		40.10%	32.20%	33.20%