Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 67,743	$ 51,600
Loans with No Related Reserve	27,537	21,504
Loans with Related Reserve	40,206	28,400
Related Reserve	5,300	3,405
Contractual Principal Balances	80,503	64,665
Average Loan Balances	47,089	41,084
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	18,550	4,250
Loans with No Related Reserve	2,598	1,395
Loans with Related Reserve	15,952	2,855
Related Reserve	2,040	505
Contractual Principal Balances	20,197	5,572
Average Loan Balances	7,535	5,053
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	15,000	14,256
Loans with No Related Reserve	8,056	7,122
Loans with Related Reserve	6,944	7,134
Related Reserve	910	934
Contractual Principal Balances	17,904	17,298
Average Loan Balances	14,941	15,083
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	9,070	8,021
Loans with No Related Reserve	7,526	6,561
Loans with Related Reserve	1,544	1,460
Related Reserve	199	215
Contractual Principal Balances	11,578	11,978
Average Loan Balances	8,191	7,910
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	6,210	4,650
Loans with No Related Reserve	4,020	2,078
Loans with Related Reserve	2,190	2,572
Related Reserve	15	15
Contractual Principal Balances	6,372	5,129
Average Loan Balances	4,155	3,339
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,478	15,065
Loans with No Related Reserve	4,220	4,348
Loans with Related Reserve	10,258	10,717
Related Reserve	1,572	1,433
Contractual Principal Balances	19,906	20,716
Average Loan Balances	9,288	7,323
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	4,435	3,662
Loans with No Related Reserve	1,117	0
Loans with Related Reserve	3,318	3,662
Related Reserve	564	303
Contractual Principal Balances	4,546	3,972
Average Loan Balances	$ 2,979	2,376
Collateral Dependent Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances		$ 49,904