Carrying values and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Carrying Reported Amount Fair Value Disclosure S A L [Member]
Financial Assets
Cash and cash equivalents	$ 166,295	$ 93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	175,568	98,411
CRA mutual fund	907	917
Federal Home Loan Bank of Boston stock	1,504	1,713
Loans held-for-sale	639	2,735
Loans receivable, net	1,057,451	1,027,738
Accrued interest receivable	5,932	6,373
Derivative financial instruments	4	4
Cash surrender value of life insurance policies	25,067	21,182
Financial Liabilities
Demand (non-interest-bearing)	392,322	310,769
Demand (interest-bearing)	220,533	218,869
Money market	328,392	278,146
Savings and other	224,286	189,776
Certificates of deposit	124,095	131,514
Deposits	1,289,628	1,129,074
Repurchase agreements	10,450	7,116
FHLBB advances	8,905	12,639
Subordinated debt	24,460	9,883
Note payable	180	208
Finance lease liability	1,631	1,673
Accrued interest payable	34	43
Estimate Of Fair Value Fair Value Disclosure S A L [Member]
Financial Assets
Cash and cash equivalents	166,295	93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	175,568	98,411
CRA mutual fund	907	917
Federal Home Loan Bank of Boston stock	1,504	1,713
Loans held-for-sale	649	2,790
Loans receivable, net	1,051,127	1,057,234
Accrued interest receivable	5,932	6,373
Derivative financial instruments	4	4
Cash surrender value of life insurance policies	25,067	21,182
Financial Liabilities
Demand (non-interest-bearing)	392,322	310,769
Demand (interest-bearing)	220,533	218,869
Money market	328,392	278,146
Savings and other	224,286	189,776
Certificates of deposit	124,946	132,875
Deposits	1,290,747	1,130,435
Repurchase agreements	10,450	7,116
FHLBB advances	9,000	12,786
Subordinated debt	24,276	10,027
Note payable	181	212
Finance lease liability	1,732	1,920
Accrued interest payable	34	43
Fair Value Inputs Level 1 S A L [Member]
Financial Assets
Cash and cash equivalents	166,295	93,162
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale
CRA mutual fund	907	917
Federal Home Loan Bank of Boston stock	1,504	1,713
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	5,932	6,373
Derivative financial instruments
Cash surrender value of life insurance policies	25,067	21,182
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
Accrued interest payable	34	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	175,568	98,411
CRA mutual fund
Federal Home Loan Bank of Boston stock
Loans held-for-sale
Loans receivable, net
Accrued interest receivable
Derivative financial instruments	4	4
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	392,322	310,769
Demand (interest-bearing)	220,533	218,869
Money market	328,392	278,146
Savings and other	224,286	189,776
Certificates of deposit	124,946	132,875
Deposits	1,290,747	1,130,435
Repurchase agreements	10,450	7,116
FHLBB advances	9,000	12,786
Subordinated debt	24,276	10,027
Note payable	181	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	649	2,790
Loans receivable, net	1,051,127	1,057,234
Accrued interest receivable
Derivative financial instruments
Cash surrender value of life insurance policies
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	1,732	1,920
Accrued interest payable