Reconciliation of Income Taxes Computed at U.S. Statutory Tax Rate to Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of income taxes amount
U.S. statutory rate	$ 43,820	$ 55,507	$ 69,107
State income taxes, net of federal tax benefit	2,097	2,199	3,103
Mining depletion	(2,751)	(2,606)	(1,162)
Section 199 Manufacturing Benefit and other	(2,851)	(2,443)	(3,734)
Income taxes	$ 40,315	$ 52,657	$ 67,314
Reconciliation of income taxes rate
U.S. statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	1.70%	1.40%	1.60%
Mining depletion	(2.20%)	(1.60%)	(0.60%)
Section 199 Manufacturing Benefit and other	(2.30%)	(1.60%)	(1.90%)
Effective Income Tax Rate, Continuing Operations, Total	32.20%	33.20%	34.10%