Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 55,852	$ 51,600
Loans with No Related Reserve	29,838	21,504
Loans with Related Reserve	26,014	28,400
Related Reserve	2,949	3,405
Contractual Principal Balances	69,513	64,665
Average Loan Balances	57,142	41,084
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,814	4,250
Loans with No Related Reserve	3,129	1,395
Loans with Related Reserve	11,685	2,855
Related Reserve	1,513	505
Contractual Principal Balances	16,967	5,572
Average Loan Balances	11,981	5,053
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,621	14,256
Loans with No Related Reserve	8,343	7,122
Loans with Related Reserve	6,278	7,134
Related Reserve	884	934
Contractual Principal Balances	17,517	17,298
Average Loan Balances	14,652	15,083
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	7,447	8,021
Loans with No Related Reserve	5,914	6,561
Loans with Related Reserve	1,533	1,460
Related Reserve	202	215
Contractual Principal Balances	9,283	11,978
Average Loan Balances	8,216	7,910
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	4,669	4,650
Loans with No Related Reserve	3,346	2,078
Loans with Related Reserve	1,323	2,572
Related Reserve	33	15
Contractual Principal Balances	4,984	5,129
Average Loan Balances	5,583	3,339
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	12,190	15,065
Loans with No Related Reserve	7,267	4,348
Loans with Related Reserve	4,923	10,717
Related Reserve	281	1,433
Contractual Principal Balances	18,385	20,716
Average Loan Balances	12,941	7,323
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	2,111	3,662
Loans with No Related Reserve	1,839	0
Loans with Related Reserve	272	3,662
Related Reserve	36	303
Contractual Principal Balances	2,377	3,972
Average Loan Balances	$ 3,769	2,376
Collateral Dependent Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances		$ 49,904