FAIR VALUE OF ASSETS AND LIABILITIES - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying value
Financial Assets
Cash and cash equivalents	$ 77,473	$ 48,486
Securities available-for-sale	85,057	78,212
CRA mutual fund	825	835
Federal Home Loan Bank of Boston stock	4,988	3,813
Loans held-for-sale	206	669
Loans receivable, net	872,796	801,703
Accrued interest receivable	3,025	2,665
Cash surrender value of life insurance policies	14,544	14,381
Financial Liabilities
Demand (non-interest-bearing)	215,149	220,536
Demand (interest-bearing)	147,120	142,575
Money market	228,918	190,953
Savings and other	172,701	144,600
Certificates of deposit	133,593	116,831
Deposits	897,481	815,495
Repurchase agreements	1,691	1,668
FHLBB advances	79,538	54,422
Subordinated debt	9,823	9,811
Note payable	297	313
Capital lease liability	3,147	1,835
Accrued interest payable	222	99
Estimated fair value
Financial Assets
Cash and cash equivalents	77,473	48,486
Securities available-for-sale	85,057	78,212
CRA mutual fund	825	835
Federal Home Loan Bank of Boston stock	4,988	3,813
Loans held-for-sale	208	669
Loans receivable, net	850,804	816,451
Accrued interest receivable	3,025	2,665
Cash surrender value of life insurance policies	14,544	14,381
Financial Liabilities
Demand (non-interest-bearing)	215,149	220,536
Demand (interest-bearing)	147,120	142,575
Money market	228,918	190,953
Savings and other	172,701	144,600
Certificates of deposit	133,032	115,290
Deposits	896,920	813,954
Repurchase agreements	1,691	1,668
FHLBB advances	79,647	54,918
Subordinated debt	10,098	10,313
Note payable	308	341
Capital lease liability	3,442	2,161
Accrued interest payable	222	99
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	77,473	48,486
Securities available-for-sale
CRA mutual fund	825	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	14,544	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	85,057	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	208	669
Loans receivable, net	850,804	816,451
Accrued interest receivable	3,025	2,665
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	215,149	220,536
Demand (interest-bearing)	147,120	142,575
Money market	228,918	190,953
Savings and other	172,701	144,600
Certificates of deposit	133,032	115,290
Deposits	896,920	813,954
Repurchase agreements	1,691	1,668
FHLBB advances	79,647	54,918
Subordinated debt	10,098	10,313
Note payable	308	341
Capital lease liability	3,442	2,161
Accrued interest payable	$ 222	$ 99