FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Carrying value
Financial Assets
Cash and cash equivalents	$ 39,262	$ 26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale, net	91,333	91,801
CRA mutual fund	900	882
Federal Home Loan Bank of Boston stock	2,733	3,242
Loans held-for-sale	580	332
Loans receivable, net	949,142	927,413
Accrued interest receivable	3,437	3,415
Cash surrender value of life insurance policies	20,714	20,580
Financial Liabilities
Demand (non-interest-bearing)	243,491	237,852
Demand (interest-bearing)	157,069	153,314
Money market	222,746	239,504
Savings and other	167,408	161,112
Certificates of deposit	174,906	127,724
Deposits	965,620	919,506
Repurchase agreements	4,929	8,530
FHLBB advances	40,932	50,887
Subordinated debt	9,865	9,859
Note payable	237	246
Finance lease liability	1,707	1,718
Accrued interest payable	297	78
Estimated fair value
Financial Assets
Cash and cash equivalents	39,262	26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale, net	91,333	91,801
CRA mutual fund	900	882
Federal Home Loan Bank of Boston stock	2,733	3,242
Loans held-for-sale	589	334
Loans receivable, net	948,157	933,287
Accrued interest receivable	3,437	3,415
Cash surrender value of life insurance policies	20,714	20,580
Financial Liabilities
Demand (non-interest-bearing)	243,491	237,852
Demand (interest-bearing)	157,069	153,314
Money market	222,746	239,504
Savings and other	167,408	161,112
Certificates of deposit	176,461	128,629
Deposits	967,175	920,411
Repurchase agreements	4,929	8,530
FHLBB advances	41,167	51,028
Subordinated debt	9,958	10,113
Note payable	241	251
Finance lease liability	1,935	1,967
Accrued interest payable	297	78
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	39,262	26,885
Interest bearing time deposits with financial institutions	750	750
Securities available-for-sale, net
CRA mutual fund	900	882
Federal Home Loan Bank of Boston stock	2,733	3,242
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	3,437	3,415
Cash surrender value of life insurance policies	20,714	20,580
Financial Liabilities
Demand (non-interest-bearing)
Demand (interest-bearing)
Money market
Savings and other
Certificates of deposit
Deposits
Repurchase agreements
FHLBB advances
Subordinated debt	9,958	10,113
Note payable
Finance lease liability
Accrued interest payable	297	78
Fair Value Measurements Using - Level 2
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale, net	91,333	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
Financial Liabilities
Demand (non-interest-bearing)	243,491	237,852
Demand (interest-bearing)	157,069	153,314
Money market	222,746	239,504
Savings and other	167,408	161,112
Certificates of deposit	176,461	128,629
Deposits	967,175	920,411
Repurchase agreements	4,929	8,530
FHLBB advances	41,167	51,028
Subordinated debt
Note payable	241	251
Finance lease liability
Accrued interest payable
Fair Value Measurements Using - Level 3
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale, net
CRA mutual fund
Federal Home Loan Bank of Boston stock
Loans held-for-sale	589	334
Loans receivable, net	948,157	933,287
Accrued interest receivable
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	1,938	1,967
Accrued interest payable