Reconciliation of Differences Between Effective Tax Rate and Federal Statutory Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Effective Tax Rates Line Items
Federal statutory tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit	4.30%	3.50%	3.50%
Non-deductible employee stock purchase plan expense	0.30%	0.30%	0.30%
Non-deductible meals and entertainment	0.30%	0.30%	0.30%
Other, net	(0.30%)	0.30%	0.10%
Effective tax rate	39.60%	39.40%	39.20%