FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Carrying value
Financial Assets
Cash and cash equivalents	$ 53,548	$ 58,445
Interest bearing time deposits with financial institutions	786
Securities available-for-sale	94,814	58,445
CRA mutual fund	881	91,818
Federal Home Loan Bank of Boston stock	2,575	836
Loans held-for-sale	506	4,496
Loans receivable, net	915,083	909,279
Accrued interest receivable	3,425	3,148
Cash surrender value of life insurance policies	20,441	14,438
Financial Liabilities
Demand (non-interest-bearing)	247,771	228,448
Demand (interest-bearing)	160,139	153,586
Money market	238,494	204,219
Savings and other	166,298	178,807
Certificates of deposit	153,476	161,679
Deposits	966,178	926,739
Repurchase agreements	8,588	4,104
FHLBB advances	37,828	67,154
Subordinated debt	9,853	9,835
Note payable	255	280
Finance lease liability	1,729	3,081
Accrued interest payable	508	237
Estimated fair value
Financial Assets
Cash and cash equivalents	53,548	58,445
Interest bearing time deposits with financial institutions	786
Securities available-for-sale	94,814	58,445
CRA mutual fund	881	91,818
Federal Home Loan Bank of Boston stock	2,575	836
Loans held-for-sale	512	4,496
Loans receivable, net	919,524	886,222
Accrued interest receivable	3,425	3,148
Cash surrender value of life insurance policies	20,441	14,438
Financial Liabilities
Demand (non-interest-bearing)	247,771	228,448
Demand (interest-bearing)	160,139	153,586
Money market	238,494	204,219
Savings and other	166,298	178,807
Certificates of deposit	154,205	162,013
Deposits	966,907	927,073
Repurchase agreements	8,588	4,104
FHLBB advances	38,010	67,231
Subordinated debt	10,072	10,006
Note payable	260	288
Finance lease liability	1,929	3,339
Accrued interest payable	508	237
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	53,548	58,445
Interest bearing time deposits with financial institutions	786
Securities available-for-sale		58,445
CRA mutual fund	881
Federal Home Loan Bank of Boston stock	2,575	836
Loans held-for-sale		4,496
Loans receivable, net
Accrued interest receivable	3,425	3,148
Cash surrender value of life insurance policies	20,441	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	10,072	10,006
Note payable
Finance lease liability
Accrued interest payable	508	237
Fair Value Measurements Using - Level 2
Financial Assets
Cash and cash equivalents
Interest bearing time deposits with financial institutions
Securities available-for-sale	94,814
CRA mutual fund		91,818
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)	247,771	228,448
Demand (interest-bearing)	160,139	153,586
Money market	238,494	204,219
Savings and other	166,298	178,807
Certificates of deposit	154,205	162,013
Deposits	966,907	927,073
Repurchase agreements	8,588	4,104
FHLBB advances	38,010	67,231
Subordinated debt
Note payable	260	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	512
Loans receivable, net	919,524	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	1,929	3,339
Accrued interest payable