Reconciliation between Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	35.00%	35.00%	35.00%
State income taxes, net of the federal tax effect	5.80%	6.00%	6.20%
Meals and entertainment	0.50%	0.50%	0.70%
Share-based compensation expense on incentive stock options and ESPP	0.40%	0.30%	1.20%
Disallowed officer's compensation	0.10%	0.10%	0.30%
Tax-exempt interest income	(0.40%)	(0.40%)	(0.90%)
Low-income housing tax credit	(1.20%)	(1.10%)	(2.40%)
Other, net	(0.90%)	0.50%	(0.80%)
Effective income tax rate	39.30%	40.90%	39.30%