Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 36,623	$ 37,710
Loans with No Related Reserve	20,097	20,983
Loans with Related Reserve	16,526	16,727
Related Reserve	2,922	2,624
Contractual Principal Balances	50,725	64,406
Average Loan Balances	40,548	42,555
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	15,421	15,710
Loans with No Related Reserve	8,618	9,034
Loans with Related Reserve	6,803	6,676
Related Reserve	915	920
Contractual Principal Balances	18,395	18,655
Average Loan Balances	15,215	15,264
Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	7,794	7,665
Loans with No Related Reserve	6,585	6,498
Loans with Related Reserve	1,209	1,167
Related Reserve	208	200
Contractual Principal Balances	9,610	9,353
Average Loan Balances	7,084	6,801
Commercial Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	5,728	6,137
Loans with No Related Reserve	947	951
Loans with Related Reserve	4,781	5,186
Related Reserve	1,477	1,168
Contractual Principal Balances	12,187	20,206
Average Loan Balances	8,060	9,391
Owner- Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	2,281	2,127
Loans with No Related Reserve	1,269	1,090
Loans with Related Reserve	1,012	1,037
Related Reserve	22	22
Contractual Principal Balances	2,488	2,947
Average Loan Balances	6,329	2,111
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	3,980	4,652
Loans with No Related Reserve	2,678	3,410
Loans with Related Reserve	1,302	1,242
Related Reserve	89	103
Contractual Principal Balances	6,626	11,826
Average Loan Balances	2,412	7,540
Construction Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	1,419	1,419
Loans with Related Reserve	1,419	1,419
Related Reserve	211	211
Contractual Principal Balances	1,419	1,419
Average Loan Balances	$ 1,448	$ 1,448