FAIR VALUE MEASUREMENTS - Carrying value and estimated fair values of financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying value
Financial Assets
Cash and cash equivalents	$ 58,445	$ 48,486
Securities available-for-sale	91,818	78,212
CRA mutual fund	836	835
Federal Home Loan Bank of Boston stock	4,496	3,813
Loans held-for-sale		669
Loans receivable, net	909,279	801,703
Accrued interest receivable	3,148	2,665
Cash surrender value of life insurance policies	14,438	14,381
Financial Liabilities
Demand (non-interest-bearing)	228,448	220,536
Demand (interest-bearing)	153,586	142,575
Money market	204,219	190,953
Savings and other	178,807	144,600
Certificates of deposit	161,679	116,831
Deposits	926,739	815,495
Repurchase agreements	4,104	1,668
FHLBB advances	67,154	54,422
Subordinated debt	9,835	9,811
Note payable	280	313
Capital lease liability	3,081	1,835
Accrued interest payable	237	99
Estimated fair value
Financial Assets
Cash and cash equivalents	58,445	48,486
Securities available-for-sale	91,818	78,212
CRA mutual fund	836	835
Federal Home Loan Bank of Boston stock	4,496	3,813
Loans held-for-sale		669
Loans receivable, net	886,222	816,451
Accrued interest receivable	3,148	2,665
Cash surrender value of life insurance policies	14,438	14,381
Financial Liabilities
Demand (non-interest-bearing)	228,448	220,536
Demand (interest-bearing)	153,586	142,575
Money market	204,219	190,953
Savings and other	178,807	144,600
Certificates of deposit	162,013	115,290
Deposits	927,073	813,954
Repurchase agreements	4,104	1,668
FHLBB advances	67,231	54,918
Subordinated debt	10,006	10,313
Note payable	288	341
Capital lease liability	3,339	2,161
Accrued interest payable	237	99
Fair Value Measurements Using - Level 1
Financial Assets
Cash and cash equivalents	58,445	48,486
Securities available-for-sale
CRA mutual fund	836	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	14,438	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	91,818	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	4,496	3,813
Loans held-for-sale		669
Loans receivable, net	886,222	816,451
Accrued interest receivable	3,148	2,665
Cash surrender value of life insurance policies
Financial Liabilities
Demand (non-interest-bearing)	228,448	220,536
Demand (interest-bearing)	153,586	142,575
Money market	204,219	190,953
Savings and other	178,807	144,600
Certificates of deposit	162,013	115,290
Deposits	927,073	813,954
Repurchase agreements	4,104	1,668
FHLBB advances	67,231	54,918
Subordinated debt	10,006	10,313
Note payable	288	341
Capital lease liability	3,339	2,161
Accrued interest payable	$ 237	$ 99