Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 68,114	$ 51,600
Loans with No Related Reserve	26,241	21,504
Loans with Related Reserve	41,873	28,400
Related Reserve	5,230	3,405
Contractual Principal Balances	81,402	64,665
Average Loan Balances	53,388	41,084
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	16,906	4,250
Loans with No Related Reserve	2,632	1,395
Loans with Related Reserve	14,274	2,855
Related Reserve	1,637	505
Contractual Principal Balances	18,726	5,572
Average Loan Balances	9,771	5,053
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,605	14,256
Loans with No Related Reserve	8,167	7,122
Loans with Related Reserve	6,438	7,134
Related Reserve	903	934
Contractual Principal Balances	17,415	17,298
Average Loan Balances	14,778	15,083
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	8,436	8,021
Loans with No Related Reserve	7,234	6,561
Loans with Related Reserve	1,202	1,460
Related Reserve	199	215
Contractual Principal Balances	11,300	11,978
Average Loan Balances	8,319	7,910
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	8,232	4,650
Loans with No Related Reserve	3,432	2,078
Loans with Related Reserve	4,800	2,572
Related Reserve	33	15
Contractual Principal Balances	8,662	5,129
Average Loan Balances	5,346	3,339
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	13,821	15,065
Loans with No Related Reserve	2,997	4,348
Loans with Related Reserve	10,824	10,717
Related Reserve	1,621	1,433
Contractual Principal Balances	19,091	20,716
Average Loan Balances	11,256	7,323
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	6,114	3,662
Loans with No Related Reserve	1,779	0
Loans with Related Reserve	4,335	3,662
Related Reserve	837	303
Contractual Principal Balances	6,208	3,972
Average Loan Balances	$ 3,918	2,376
Collateral Dependent Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances		$ 49,904