FAIR VALUE MEASUREMENTS - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Carrying value
Financial Assets
Cash and cash equivalents	$ 26,885	$ 58,445
Interest bearing time deposits with financial institutions	750
Securities available-for-sale	91,801	91,818
CRA mutual fund	882	836
Federal Home Loan Bank of Boston stock	3,242	4,496
Loans held-for-sale	332
Loans receivable, net	927,413	909,279
Accrued interest receivable	3,415	3,148
Cash surrender value of life insurance policies	20,580	14,438
Financial Liabilities
Demand (non-interest-bearing)	237,852	228,448
Demand (interest-bearing)	153,314	153,586
Money market	239,504	204,219
Savings and other	161,112	178,807
Certificates of deposit	127,724	161,679
Deposits	919,506	926,739
Repurchase agreements	8,530	4,104
FHLBB advances	50,887	67,154
Subordinated debt	9,859	9,835
Note payable	246	280
Finance lease liability	1,718	3,081
Accrued interest payable	78	237
Estimated fair value
Financial Assets
Cash and cash equivalents	26,885	58,445
Interest bearing time deposits with financial institutions	750
Securities available-for-sale	91,801	91,818
CRA mutual fund	882	836
Federal Home Loan Bank of Boston stock	3,242	4,496
Loans held-for-sale	334
Loans receivable, net	933,287	886,222
Accrued interest receivable	3,415	3,148
Cash surrender value of life insurance policies	20,580	14,438
Financial Liabilities
Demand (non-interest-bearing)	237,852	228,448
Demand (interest-bearing)	153,314	153,586
Money market	239,504	204,219
Savings and other	161,112	178,807
Certificates of deposit	128,629	162,013
Deposits	920,411	927,073
Repurchase agreements	8,530	4,104
FHLBB advances	51,028	67,231
Subordinated debt	10,113	10,006
Note payable	251	288
Finance lease liability	1,967	3,339
Accrued interest payable	78	237
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	26,885	58,445
Interest bearing time deposits with financial institutions	750
Securities available-for-sale
CRA mutual fund	882	836
Federal Home Loan Bank of Boston stock	3,242	4,496
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	3,415	3,148
Cash surrender value of life insurance policies	20,580	14,438
Financial Liabilities
Demand (non-interest-bearing)
Demand (interest-bearing)
Money market
Savings and other
Certificates of deposit
Deposits
Repurchase agreements
FHLBB advances
Subordinated debt	10,113	10,006
Note payable
Finance lease liability
Accrued interest payable	78	237
Fair Value Measurements Using - Level 2
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale	91,801	91,818
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)	237,852	228,448
Demand (interest-bearing)	153,314	153,586
Money market	239,504	204,219
Savings and other	161,112	178,807
Certificates of deposit	128,629	162,013
Deposits	920,411	927,073
Repurchase agreements	8,530	4,104
FHLBB advances	51,028	67,231
Subordinated debt
Note payable	251	288
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
CRA mutual fund
Federal Home Loan Bank of Boston stock
Loans held-for-sale	334
Loans receivable, net	933,287	886,222
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	1,967	3,339
Accrued interest payable