composition of securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Cost of Security Sold or Transferred, Method, Specific Identification [Member]
Available-for-sale
U.S. Treasury	$ 10,397
U.S. Government Agency notes	32,093	$ 7,735
Municipal bonds	38,525	25,831
U.S. Government agencies and U.S. Government - sponsored enterprises	67,059	35,240
U.S. Government agencies	14,844	17,054
Corporate bonds	11,000	8,750
Total securities available-for-sale	173,918	94,610
Non-marketable securities
Federal Home Loan Bank of Boston stock	1,504	1,713
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Available-for-sale
U.S. Treasury
U.S. Government Agency notes	280	153
Municipal bonds	1,331	1,787
U.S. Government agencies and U.S. Government - sponsored enterprises	818	1,376
U.S. Government agencies	208	400
Corporate bonds	197	166
Total securities available-for-sale	2,834	3,882
Non-marketable securities
Federal Home Loan Bank of Boston stock
Accumulated Net Unrealized Investment Gain Loss 1 [Member]
Available-for-sale
U.S. Treasury	95
U.S. Government Agency notes	99	37
Municipal bonds	251	1
U.S. Government agencies and U.S. Government - sponsored enterprises	698	43
U.S. Government agencies	41
Corporate bonds
Total securities available-for-sale	1,184	81
Non-marketable securities
Federal Home Loan Bank of Boston stock
Securities Financing Transaction, Fair Value [Member]
Available-for-sale
U.S. Treasury	10,302
U.S. Government Agency notes	32,274	7,851
Municipal bonds	39,605	27,617
U.S. Government agencies and U.S. Government - sponsored enterprises	67,179	36,573
U.S. Government agencies	15,011	17,454
Corporate bonds	11,197	8,916
Total securities available-for-sale	175,568	98,411
CRA mutual fund	907	917
Non-marketable securities
Federal Home Loan Bank of Boston stock	$ 1,504	$ 1,713