FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Carrying value
Financial Assets
Cash and cash equivalents	$ 44,979	$ 48,486
Securities available-for-sale	79,906	78,212
CRA mutual fund	826	835
Federal Home Loan Bank of Boston stock	4,146	3,813
Loans held-for-sale		669
Loans receivable, net	830,370	801,703
Accrued interest receivable	2,704	2,665
Cash surrender value of life insurance policies	14,462	14,381
Financial Liabilities
Demand (non-interest-bearing)	220,796	220,536
Demand (interest-bearing)	146,312	142,575
Money market	185,955	190,953
Savings and other	155,630	144,600
Certificates of deposit	123,144	116,831
Deposits	831,838	815,495
Repurchase agreements	3,962	1,668
FHLBB advances	62,480	54,422
Subordinated debt	9,817	9,811
Note payable	305	313
Capital lease liability	3,179	1,835
Accrued interest payable	307	99
Estimated fair value
Financial Assets
Cash and cash equivalents	44,979	48,486
Securities available-for-sale	79,906	78,212
CRA mutual fund	826	835
Federal Home Loan Bank of Boston stock	4,146	3,813
Loans held-for-sale		669
Loans receivable, net	813,423	816,451
Accrued interest receivable	2,704	2,665
Cash surrender value of life insurance policies	14,462	14,381
Financial Liabilities
Demand (non-interest-bearing)	220,796	220,536
Demand (interest-bearing)	146,312	142,575
Money market	185,955	190,953
Savings and other	155,630	144,600
Certificates of deposit	120,890	115,290
Deposits	831,100	813,954
Repurchase agreements	3,962	1,668
FHLBB advances	62,697	54,918
Subordinated debt	10,193	10,313
Note payable	317	341
Capital lease liability	3,493	2,161
Accrued interest payable	307	99
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	44,979	48,486
Securities available-for-sale
CRA mutual fund	826	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	14,462	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	79,906	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	4,146	3,813
Loans held-for-sale		669
Loans receivable, net	813,423	816,451
Accrued interest receivable	2,704	2,665
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	220,796	220,536
Demand (interest-bearing)	146,312	142,575
Money market	185,955	190,953
Savings and other	155,630	144,600
Certificates of deposit	120,890	115,290
Deposits	831,100	813,954
Repurchase agreements	3,962	1,668
FHLBB advances	62,697	54,918
Subordinated debt	10,193	10,313
Note payable	317	341
Capital lease liability	3,493	2,161
Accrued interest payable	$ 307	$ 99