Allowance for Loan Losses and Credit Quality Information - Analysis of Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	$ 49,352	$ 62,106
Loans with No Related Reserve	30,455	32,172
Loans with Related Reserve (2)	18,897	29,934
Related Reserve	2,258	5,017
Contractual Principal Balances (2)	71,500	78,982
Average Loan Balances	56,936	60,743
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	6,065	5,374
Loans with No Related Reserve	4,406	3,654
Loans with Related Reserve (2)	1,659	1,720
Related Reserve	92	12
Contractual Principal Balances (2)	6,337	5,717
Average Loan Balances	5,378	5,827
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	5,679	7,598
Loans with No Related Reserve	4,083	4,487
Loans with Related Reserve (2)	1,596	3,111
Related Reserve	79	112
Contractual Principal Balances (2)	15,372	16,658
Average Loan Balances	7,438	12,630
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	3,530	6,292
Loans with No Related Reserve	0	6,023
Loans with Related Reserve (2)	3,530	269
Related Reserve	458	33
Contractual Principal Balances (2)	5,082	6,800
Average Loan Balances	5,091	4,523
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	14,841	20,842
Loans with No Related Reserve	8,625	3,422
Loans with Related Reserve (2)	6,216	17,420
Related Reserve	878	3,861
Contractual Principal Balances (2)	22,365	23,815
Average Loan Balances	18,484	15,072
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	11,321	14,181
Loans with No Related Reserve	6,442	8,282
Loans with Related Reserve (2)	4,879	5,899
Related Reserve	581	796
Contractual Principal Balances (2)	13,771	17,015
Average Loan Balances	12,589	14,533
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ending Loan Balances	7,916	7,819
Loans with No Related Reserve	6,899	6,304
Loans with Related Reserve (2)	1,017	1,515
Related Reserve	170	203
Contractual Principal Balances (2)	8,573	8,977
Average Loan Balances	$ 7,956	$ 8,158