FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying value
Financial Assets
Cash and cash equivalents	$ 48,513	$ 58,445
Securities available-for-sale	100,149	91,818
CRA mutual fund	869	836
Federal Home Loan Bank of Boston stock	2,839	4,496
Loans held-for-sale	403
Loans receivable, net	910,573	909,279
Accrued interest receivable	3,439	3,148
Cash surrender value of life insurance policies	15,355	14,438
Financial Liabilities
Demand (non-interest-bearing)	231,019	228,448
Demand (interest-bearing)	156,687	153,586
Money market	231,987	204,219
Savings and other	158,809	178,807
Certificates of deposit	172,221	161,679
Deposits	950,723	926,739
Repurchase agreements	6,308	4,104
FHLBB advances	32,769	67,154
Subordinated debt	9,847	9,835
Note payable	263	280
Capital lease liability	3,011	3,081
Accrued interest payable	576	237
Estimated fair value
Financial Assets
Cash and cash equivalents	48,513	58,445
Securities available-for-sale	100,149	91,818
CRA mutual fund	869	836
Federal Home Loan Bank of Boston stock	2,839	4,496
Loans held-for-sale	403
Loans receivable, net	910,149	886,222
Accrued interest receivable	3,439	3,148
Cash surrender value of life insurance policies	15,355	14,438
Financial Liabilities
Demand (non-interest-bearing)	231,019	228,448
Demand (interest-bearing)	156,687	153,586
Money market	231,987	204,219
Savings and other	158,809	178,807
Certificates of deposit	172,865	162,013
Deposits	951,397	927,073
Repurchase agreements	6,308	4,104
FHLBB advances	33,057	67,231
Subordinated debt	10,121	10,006
Note payable	269	288
Capital lease liability	3,225	3,339
Accrued interest payable	576	237
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	48,513	58,445
Securities available-for-sale
CRA mutual fund	869	836
Federal Home Loan Bank of Boston stock	2,839
Loans held-for-sale
Loans receivable, net
Accrued interest receivable	3,439
Cash surrender value of life insurance policies	15,355	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	10,121
Note payable
Capital lease liability
Accrued interest payable	576
Fair Value Measurements Using - Level 2
Financial Assets
Cash and cash equivalents
Securities available-for-sale	100,149	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)	231,019
Demand (interest-bearing)	156,687
Money market	231,987
Savings and other	158,809
Certificates of deposit	172,865
Deposits	951,397
Repurchase agreements	6,308
FHLBB advances	33,057
Subordinated debt
Note payable	269
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		4,496
Loans held-for-sale	403
Loans receivable, net	910,149	886,222
Accrued interest receivable		3,148
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)		228,448
Demand (interest-bearing)		153,586
Money market		204,219
Savings and other		178,807
Certificates of deposit		162,013
Deposits		927,073
Repurchase agreements		4,104
FHLBB advances		67,231
Subordinated debt		10,006
Note payable		288
Capital lease liability	3,225	3,339
Accrued interest payable		$ 237