Reconciliation of Income Taxes Computed at the U.S. Statutory Tax Rate to the Effective Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of income taxes amount
U.S. statutory rate	$ 69,107	$ 41,772	$ 27,928
State income taxes, net of federal tax benefit	3,103	2,148	1,351
Mining depletion	(1,162)	(1,227)	(898)
Section 199 Manufacturing Benefit, ETI Exclusion and other	(3,734)	(2,060)	(1,397)
Income taxes	$ 67,314	$ 40,633	$ 26,984
Reconciliation of income taxes rate
U.S. statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	1.60%	1.80%	1.70%
Mining depletion	(0.60%)	(1.00%)	(1.10%)
Section 199 Manufacturing Benefit, ETI Exclusion and other	(1.90%)	(1.80%)	(1.80%)
Effective Income Tax Rate, Continuing Operations, Total	34.10%	34.00%	33.80%