FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Carrying value
Financial Assets
Cash and cash equivalents	$ 42,220	$ 48,486
Securities available-for-sale	95,780	78,212
CRA mutual fund	823	835
Federal Home Loan Bank of Boston stock	4,988	3,813
Loans held-for-sale	589	669
Loans receivable, net	898,625	801,703
Accrued interest receivable	3,317	2,665
Cash surrender value of life insurance policies	14,627	14,381
Financial Liabilities
Demand (non-interest-bearing)	233,935	220,536
Demand (interest-bearing)	151,830	142,575
Money market	202,308	190,953
Savings and other	176,415	144,600
Certificates of deposit	137,673	116,831
Deposits	902,161	815,495
Repurchase agreements	6,658	1,668
FHLBB advances	67,596	54,422
Subordinated debt	9,829	9,811
Note payable	289	313
Capital lease liability	3,114	1,835
Accrued interest payable	382	99
Estimated fair value
Financial Assets
Cash and cash equivalents	42,220	48,486
Securities available-for-sale	95,780	78,212
CRA mutual fund	823	835
Federal Home Loan Bank of Boston stock	4,988	3,813
Loans held-for-sale	599	669
Loans receivable, net	873,397	816,451
Accrued interest receivable	3,317	2,665
Cash surrender value of life insurance policies	14,627	14,381
Financial Liabilities
Demand (non-interest-bearing)	233,935	220,536
Demand (interest-bearing)	151,830	142,575
Money market	202,308	190,953
Savings and other	176,415	144,600
Certificates of deposit	137,648	115,290
Deposits	902,136	813,954
Repurchase agreements	6,658	1,668
FHLBB advances	67,589	54,918
Subordinated debt	10,063	10,313
Note payable	297	341
Capital lease liability	3,391	2,161
Accrued interest payable	382	99
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	42,220	48,486
Securities available-for-sale
CRA mutual fund	823	835
Federal Home Loan Bank of Boston stock
Loans held-for-sale
Loans receivable, net
Accrued interest receivable
Cash surrender value of life insurance policies	14,627	14,381
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	95,780	78,212
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	4,988	3,813
Loans held-for-sale	599	669
Loans receivable, net	873,397	816,451
Accrued interest receivable	3,317	2,665
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	233,935	220,536
Demand (interest-bearing)	151,830	142,575
Money market	202,308	190,953
Savings and other	176,415	144,600
Certificates of deposit	137,648	115,290
Deposits	902,136	813,954
Repurchase agreements	6,658	1,668
FHLBB advances	67,589	54,918
Subordinated debt	10,063	10,313
Note payable	297	341
Capital lease liability	3,391	2,161
Accrued interest payable	$ 382	$ 99