Carrying values and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Carrying Reported Amount Fair Value Disclosure S A L [Member]
Financial Assets
Cash and cash equivalents	$ 159,196	$ 93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	127,343	98,411
CRA mutual fund	904	917
Federal Home Loan Bank of Boston stock	1,713	1,713
Loans held-for-sale	2,313	2,735
Loans receivable, net	1,041,185	1,027,738
Accrued interest receivable	6,237	6,373
Cash surrender value of life insurance policies	21,307	21,182
Derivative financial instruments	7	4
Financial Liabilities
Demand (non-interest-bearing)	334,638	310,769
Demand (interest-bearing)	229,200	218,869
Money market	311,971	278,146
Savings and other	207,109	189,776
Certificates of deposit	128,253	131,514
Deposits	1,211,171	1,129,074
Repurchase agreements	8,687	7,116
FHLBB advances	11,396	12,639
Subordinated debt	34,305	9,883
Note payable	197	208
Finance lease liability	1,658	1,673
Accrued interest payable	37	43
Estimate Of Fair Value Fair Value Disclosure S A L [Member]
Financial Assets
Cash and cash equivalents	159,196	93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	127,343	98,411
CRA mutual fund	904	917
Federal Home Loan Bank of Boston stock	1,713	1,713
Loans held-for-sale	2,352	2,790
Loans receivable, net	1,043,432	1,057,234
Accrued interest receivable	6,237	6,373
Cash surrender value of life insurance policies	21,307	21,182
Derivative financial instruments	7	4
Financial Liabilities
Demand (non-interest-bearing)	334,638	310,769
Demand (interest-bearing)	229,200	218,869
Money market	311,971	278,146
Savings and other	207,109	189,776
Certificates of deposit	129,440	132,875
Deposits	1,212,358	1,130,435
Repurchase agreements	8,687	7,116
FHLBB advances	11,525	12,786
Subordinated debt	35,041	10,027
Note payable	202	212
Finance lease liability	1,801	1,920
Accrued interest payable	37	43
Fair Value Inputs Level 1 S A L [Member]
Financial Assets
Cash and cash equivalents	159,196	93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale
CRA mutual fund	904	917
Federal Home Loan Bank of Boston stock	1,713	1,713
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	6,237	6,373
Cash surrender value of life insurance policies	21,307	21,182
Derivative financial instruments
Financial Liabilities
Demand (non-interest-bearing)
Demand (interest-bearing)
Money market
Savings and other
Certificates of deposit
Deposits
Repurchase agreements
FHLBB advances
Subordinated debt	35,041	10,027
Note payable
Finance lease liability
Accrued interest payable	37	43
Fair Value Inputs Level 2 S A L [Member]
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale	127,343	98,411
CRA mutual fund
Federal Home Loan Bank of Boston stock
Loans held-for-sale
Loans receivable, net
Accrued interest receivable
Cash surrender value of life insurance policies
Derivative financial instruments	7	4
Financial Liabilities
Demand (non-interest-bearing)	334,638	310,769
Demand (interest-bearing)	229,200	218,869
Money market	311,971	278,146
Savings and other	207,109	189,776
Certificates of deposit	129,440	132,875
Deposits	1,212,358	1,130,435
Repurchase agreements	8,687	7,116
FHLBB advances	11,525	12,786
Subordinated debt
Note payable	202	212
Finance lease liability
Accrued interest payable
Fair Value Inputs Level 3 S A L [Member]
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale
CRA mutual fund
Federal Home Loan Bank of Boston stock
Loans held-for-sale	2,352	2,790
Loans receivable, net	1,043,432	1,057,234
Accrued interest receivable
Cash surrender value of life insurance policies
Derivative financial instruments
Financial Liabilities
Demand (non-interest-bearing)
Demand (interest-bearing)
Money market
Savings and other
Certificates of deposit
Deposits
Repurchase agreements
FHLBB advances
Subordinated debt
Note payable
Finance lease liability	1,801	1,920
Accrued interest payable