Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 62,106	$ 49,904
Loans with No Related Reserve	32,172	21,504
Loan with Related Reserve	29,934	28,400
Related Reserve	5,017	3,405
Contractual Principal Balance	78,982	64,665
Average Loan Balances	60,743	41,084
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	20,842	4,250
Loans with No Related Reserve	3,422	1,395
Loan with Related Reserve	17,420	2,855
Related Reserve	3,861	505
Contractual Principal Balance	23,815	5,572
Average Loan Balances	15,072	5,053
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,181	14,256
Loans with No Related Reserve	8,282	7,122
Loan with Related Reserve	5,899	7,134
Related Reserve	796	934
Contractual Principal Balance	17,015	17,298
Average Loan Balances	14,533	15,083
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	7,819	8,021
Loans with No Related Reserve	6,304	6,561
Loan with Related Reserve	1,515	1,460
Related Reserve	203	215
Contractual Principal Balance	8,977	11,978
Average Loan Balances	8,158	7,910
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	5,374	4,650
Loans with No Related Reserve	3,654	2,078
Loan with Related Reserve	1,720	2,572
Related Reserve	12	15
Contractual Principal Balance	5,717	5,129
Average Loan Balances	5,827	3,339
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	7,598	15,065
Loans with No Related Reserve	4,487	4,348
Loan with Related Reserve	3,111	10,717
Related Reserve	112	1,433
Contractual Principal Balance	16,658	20,716
Average Loan Balances	12,630	7,323
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	6,292	3,662
Loans with No Related Reserve	6,023	0
Loan with Related Reserve	269	3,662
Related Reserve	33	303
Contractual Principal Balance	6,800	3,972
Average Loan Balances	$ 4,523	$ 2,376