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, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of income taxes amount
U.S. statutory rate	$ 55,507	$ 69,107	$ 41,772
State income taxes, net of federal tax benefit	2,199	3,103	2,148
Mining depletion	(2,606)	(1,162)	(1,227)
Section 199 Manufacturing Benefit and other	(2,443)	(3,734)	(2,060)
Income taxes	$ 52,657	$ 67,314	$ 40,633
Reconciliation of income taxes rate
U.S. statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	1.40%	1.60%	1.80%
Mining depletion	(1.60%)	(0.60%)	(1.00%)
Section 199 Manufacturing Benefit and other	(1.60%)	(1.90%)	(1.80%)
Effective Income Tax Rate, Continuing Operations, Total	33.20%	34.10%	34.00%