FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying value
Financial Assets
Cash and cash equivalents	$ 48,629	$ 58,445
Securities available-for-sale	98,255	91,818
CRA mutual fund	852	836
Federal Home Loan Bank of Boston stock	3,372	4,496
Loans held-for-sale
Loans receivable, net	911,188	909,279
Accrued interest receivable	3,411	3,148
Cash surrender value of life insurance policies	15,267	14,438
Financial Liabilities
Demand (non-interest-bearing)	219,168	228,448
Demand (interest-bearing)	157,123	153,586
Money market	203,309	204,219
Savings and other	190,011	178,807
Certificates of deposit	172,358	161,679
Deposits	941,969	926,739
Repurchase agreements	2,951	4,104
FHLBB advances	47,712	67,154
Subordinated debt	9,841	9,835
Note payable	272	280
Capital lease liability	3,046	3,081
Accrued interest payable	578	237
Estimated fair value
Financial Assets
Cash and cash equivalents	48,629	58,445
Securities available-for-sale	98,255	91,818
CRA mutual fund	852	836
Federal Home Loan Bank of Boston stock	3,372	4,496
Loans held-for-sale
Loans receivable, net	902,084	886,222
Accrued interest receivable	3,411	3,148
Cash surrender value of life insurance policies	15,267	14,438
Financial Liabilities
Demand (non-interest-bearing)	219,168	228,448
Demand (interest-bearing)	157,123	153,586
Money market	203,309	204,219
Savings and other	190,011	178,807
Certificates of deposit	172,895	162,013
Deposits	942,506	927,073
Repurchase agreements	2,951	4,104
FHLBB advances	47,925	67,231
Subordinated debt	10,115	10,006
Note payable	279	288
Capital lease liability	3,282	3,339
Accrued interest payable	578	237
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	48,629	58,445
Securities available-for-sale
CRA mutual fund	852	836
Federal Home Loan Bank of Boston stock
Loans held-for-sale
Loans receivable, net
Accrued interest receivable
Cash surrender value of life insurance policies	15,267	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
Note payable
Capital lease liability
Accrued interest payable
Fair Value Measurements Using - Level 2
Financial Assets
Cash and cash equivalents
Securities available-for-sale	98,255	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)
Demand (interest-bearing)
Money market
Savings and other
Certificates of deposit
Deposits
Repurchase agreements
FHLBB advances
Subordinated debt
Note payable
Capital lease liability
Accrued interest payable
Fair Value Measurements Using - Level 3
Financial Assets
Cash and cash equivalents
Securities available-for-sale
CRA mutual fund
Federal Home Loan Bank of Boston stock	3,372	4,496
Loans held-for-sale
Loans receivable, net	902,084	886,222
Accrued interest receivable	3,411	3,148
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	219,168	228,448
Demand (interest-bearing)	157,123	153,586
Money market	203,309	204,219
Savings and other	190,011	178,807
Certificates of deposit	172,895	162,013
Deposits	942,506	927,073
Repurchase agreements	2,951	4,104
FHLBB advances	47,925	67,231
Subordinated debt	10,115	10,006
Note payable	279	288
Capital lease liability	3,282	3,339
Accrued interest payable	$ 578	$ 237