Loans - Narrative (Details)	12 Months Ended		24 Months Ended
	Dec. 31, 2021 USD ($) loan_modification_request loan	Dec. 31, 2020 USD ($) loan	Dec. 31, 2021 USD ($) loan	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net deferred loan costs included in loan balances	$ 7,890,000	$ 6,931,000	$ 7,890,000
Loans used to collateralize borrowings from the Federal Home Loan Bank of Boston	364,968,000	378,183,000	364,968,000
Commercial, construction and home equity loans used to collateralize unused line of credit at the Federal Reserve Bank of Boston	$ 295,090,000	$ 259,599,000	295,090,000
Number of loans | loan	60	74
Loans	$ 1,647,649,000	$ 1,476,761,000	1,647,649,000	$ 1,297,075,000
Nonaccrual loans	5,602,000	6,721,000	5,602,000
Interest income which would have been recognized on these loans, if interest had been accrued	345,000	558,000
Loans more than 90 days past due accruing interest	32,000	1,505,000	32,000
Loans to directors, officers and employees	42,784,000	36,880,000	42,784,000
TDR recorded investment	$ 8,341,000	$ 11,534,000	$ 8,341,000
Number of loans | loan	5	14	5
TDR recorded investment more than 30 days past due	$ 349,000	$ 1,577,000	$ 349,000
Number of loans placed on TDR status in previous 12 months | loan	1	1
Number of loans | loan	4	3
Number of TDR loans involved in bankruptcy (loan) | loan	8		8
Loans classified as TDRs that are involved in bankruptcy	$ 950,000		$ 950,000
Number of loans classified as TDRs that were on non-accrual status | loan	20		20
Loans classified as TDRs that were on non-accrual status	$ 1,894,000		$ 1,894,000
Loans classified as TDRs that are involved in foreclosure	$ 0		$ 0
Number of mortgage loans in the process of foreclosure | loan	4	11	4
Mortgage loans in the process of foreclosure	$ 367,000	$ 1,109,000	$ 367,000
Original or Subsequent Modification | Exemption from Troubled Debt Restructure Designation, CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,854,000		$ 2,854,000
Percentage of loans receivable, by type	0.0017		0.0017
Out of Modification | Exemption from Troubled Debt Restructure Designation, CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 215,167,000		$ 215,167,000
Past due rate	0.41%		0.41%
Out of Modification | Exemption from Troubled Debt Restructure Designation, CARES Act | All Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 875,000		$ 875,000
PPP1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	1,053	1,718
Loans	$ 8,000	$ 97,755,000	$ 8,000
Percentage of loans receivable, by type	0.173		0.173
TDR recorded investment	$ 284,813,000		$ 284,813,000
PPP2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	1,263
Loans	$ 22,025,000		22,025,000
Cumulative amount granted			$ 52,053,000