COMMITMENTS AND CONTINGENCIES - Loan commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Loan commitments
Commitments to grant loans	$ 7,890	$ 16,298
Home equity loans and lines of credit
Loan commitments
Unadvanced funds on loan	10,349	8,513
Commercial lines of credit
Loan commitments
Unadvanced funds on loan	18,218	14,713
Construction loans
Loan commitments
Unadvanced funds on loan	16,264	12,423
Other unsecured personal lines of credit
Loan commitments
Unadvanced funds on loan	274	287
Commercial real estate
Loan commitments
Unadvanced funds on loan	5,237	5,618
Standby letters of credit
Loan commitments
Standby letters of credit	$ 227	$ 62
Standby letters of credit | Maximum
Loan commitments
Term of letters of credit	1 year