Reconcilements Between Statutory Federal Income Tax Rate And Effective Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Disclosure Reconcilements Between Statutory Federal Income Tax Rate and Effective Tax Rate [Line Items]
Income tax expense at federal statutory rate	$ 19,110	$ 17,481	$ 11,399
Increase (decrease) resulting from:
Income tax expense	18,045	15,845	9,049
Tax exempt income, net	(2,927)	(2,945)	(2,411)
Bank-owned life insurance	(916)	(923)	(980)
State income taxes, net of federal income tax benefits	2,424	2,187	906
Other, net	354	45	135
Total income tax expense (benefit) and rate	$ 18,045	$ 15,845	$ 9,049
Income tax expense at federal statutory rate	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax exempt income, net	(5.40%)	(5.90%)	(7.40%)
Bank-owned life insurance	(1.70%)	(1.90%)	(3.00%)
State income taxes, net of federal income tax benefits	4.40%	4.40%	2.80%
Other, net	0.70%	0.10%	0.40%
Total income tax expense (benefit) and rate	33.00%	31.70%	27.80%