Carrying values and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Inputs, Level 1 [Member]
Financial Assets
Cash and cash equivalents	$ 93,162	$ 26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale
CRA mutual fund	917	882
Federal Home Loan Bank of Boston stock	1,713	3,242
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	6,373	3,415
Cash surrender value of life insurance policies	21,182	20,580
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	10,027	10,113
Note payable
Finance lease liability
Accrued interest payable	43	78
Fair Value, Inputs, Level 2 [Member]
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale	98,411	91,801
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	4
Financial Liabilities
Demand (non-interest-bearing)	310,769	237,852
Demand (interest-bearing)	218,869	153,314
Money market	278,146	239,504
Savings and other	189,776	161,112
Certificates of deposit	132,875	128,629
Deposits	1,130,435	920,411
Repurchase agreements	7,116	8,530
FHLBB advances	12,786	51,028
Subordinated debt
Note payable	212	251
Finance lease liability
Accrued interest payable
Fair Value, Inputs, Level 3 [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	2,790	334
Loans receivable, net	1,057,234	933,287
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	1,920	1,967
Accrued interest payable
Reported Value Measurement [Member]
Financial Assets
Cash and cash equivalents	93,162	26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	98,411	91,801
CRA mutual fund	917	882
Federal Home Loan Bank of Boston stock	1,713	3,242
Loans held-for-sale	2,735	332
Loans receivable, net	1,027,738	927,413
Accrued interest receivable	6,373	3,415
Cash surrender value of life insurance policies	21,182	20,580
Derivative financial instruments	4
Financial Liabilities
Demand (non-interest-bearing)	310,769	237,852
Demand (interest-bearing)	218,869	153,314
Money market	278,146	239,504
Savings and other	189,776	161,112
Certificates of deposit	131,514	127,724
Deposits	1,129,074	919,506
Repurchase agreements	7,116	8,530
FHLBB advances	12,639	50,887
Subordinated debt	9,883	9,859
Note payable	208	246
Finance lease liability	1,673	1,718
Accrued interest payable	43	78
Estimate of Fair Value Measurement [Member]
Financial Assets
Cash and cash equivalents	93,162	26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale	98,411	91,801
CRA mutual fund	917	882
Federal Home Loan Bank of Boston stock	1,713	3,242
Loans held-for-sale	2,790	334
Loans receivable, net	1,057,234	933,287
Accrued interest receivable	6,373	3,415
Cash surrender value of life insurance policies	21,182	20,580
Derivative financial instruments	4
Financial Liabilities
Demand (non-interest-bearing)	310,769	237,852
Demand (interest-bearing)	218,869	153,314
Money market	278,146	239,504
Savings and other	189,776	161,112
Certificates of deposit	132,875	128,629
Deposits	1,130,435	920,411
Repurchase agreements	7,116	8,530
FHLBB advances	12,786	51,028
Subordinated debt	10,027	10,113
Note payable	212	251
Finance lease liability	1,920	1,967
Accrued interest payable	$ 43	$ 78