Analysis of Impaired Loans by Class (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	$ 36,277	$ 27,372
Recorded Investment with No Allowance	18,463	13,325
Recorded Investment with Allowance	8,867	10,227
Total Recorded Investment	27,330	23,552
Related Allowance	2,658	3,930
Average Recorded Investment	25,916	17,393
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	21,687	14,368
Recorded Investment with No Allowance	7,455	2,940
Recorded Investment with Allowance	5,984	8,121
Total Recorded Investment	13,439	11,061
Related Allowance	1,591	3,662
Average Recorded Investment	12,844	8,038
Real estate - construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	263	90
Recorded Investment with No Allowance	262	50
Total Recorded Investment	262	50
Average Recorded Investment	78	15
Real estate - commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	11,784	9,323
Recorded Investment with No Allowance	9,076	7,983
Recorded Investment with Allowance	2,515	1,247
Total Recorded Investment	11,591	9,230
Related Allowance	981	226
Average Recorded Investment	10,158	7,000
Real estate - residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	2,498	3,568
Recorded Investment with No Allowance	1,625	2,329
Recorded Investment with Allowance	368	859
Total Recorded Investment	1,993	3,188
Related Allowance	86	42
Average Recorded Investment	2,794	2,312
Consumer - other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance	45	23
Recorded Investment with No Allowance	45	23
Total Recorded Investment	45	23
Average Recorded Investment	$ 42	$ 28