Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 49,329	$ 57,855
Loans with No Specific Reserve	25,133	38,707
Loans with Specific Reserve	24,196	19,148
Related Specific Reserve	5,951	4,951
Contractual Principal Balances	88,962	98,657
Average Loan Balances	53,180	64,752
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	5,833	4,861
Loans with No Specific Reserve	2,288	1,598
Loans with Specific Reserve	3,545	3,263
Related Specific Reserve	2,004	2,100
Contractual Principal Balances	13,728	12,060
Average Loan Balances	5,062	4,993
Owner - Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	12,568	14,001
Loans with No Specific Reserve	5,723	13,827
Loans with Specific Reserve	6,845	174
Related Specific Reserve	945	1
Contractual Principal Balances	14,911	18,658
Average Loan Balances	13,167	16,856
Commercial Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	8,894	12,634
Loans with No Specific Reserve	2,896	5,422
Loans with Specific Reserve	5,998	7,212
Related Specific Reserve	1,995	1,887
Contractual Principal Balances	19,494	22,192
Average Loan Balances	9,887	10,233
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	274	1,547
Loans with No Specific Reserve	274	1,172
Loans with Specific Reserve		375
Related Specific Reserve		28
Contractual Principal Balances	16,012	17,711
Average Loan Balances	1,261	11,239
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	17,360	18,483
Loans with No Specific Reserve	9,680	11,053
Loans with Specific Reserve	7,680	7,430
Related Specific Reserve	995	919
Contractual Principal Balances	19,726	20,771
Average Loan Balances	18,091	16,917
Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	4,400	6,329
Loans with No Specific Reserve	4,272	5,635
Loans with Specific Reserve	128	694
Related Specific Reserve	12	16
Contractual Principal Balances	5,091	7,265
Average Loan Balances	$ 5,712	$ 4,514