Summary of Activity in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 23,262	$ 22,989	$ 23,011	$ 22,293	$ 22,293
Loan charge-offs	(1,416)	(1,400)	(3,366)	(3,417)	(5,025)
Recoveries on loans previously charged off	146	235	530	865	1,002
Net charge-offs	(1,270)	(1,165)	(2,836)	(2,552)
Provision for loan losses	859	1,187	2,676	3,270	4,741
Ending balance	$ 22,851	$ 23,011	$ 22,851	$ 23,011	$ 23,011