MORTGAGE NOTES PAYABLE (Mortgage by property) (Details)				1 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 20, 2019 USD ($) item	May 31, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2021 USD ($) item	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 item	Sep. 30, 2015 USD ($)
MORTGAGE NOTES PAYABLE
Line of credit					$ 0	$ 17,000,000
Residences at CaptainParkers LLC Residential Apartments Lexington, Massachusetts
MORTGAGE NOTES PAYABLE
Loan amount			$ 20,750,000
Interest rate (as a percent)			4.05%
Loan balance			$ 20,071,000
Mortgage prepayment penalties			$ 202,000
57 Mill Street
MORTGAGE NOTES PAYABLE
Number of units | item		181
Loan amount		$ 31,000,000
Purchase price of real estate properties		59,550,000
Cash from reserves		10,550,000
Line of credit		18,000,000
Closing Costs		237,000
Purchase price allocated to the value of the in-place leases		1,282,000
Purchase price allocated to the value of the tenant relationships		$ 136,000
Amortization period of value of the in-place leases		12 months
Amortization period of value of the tenant relationships		36 months
Hamilton Essex 81
MORTGAGE NOTES PAYABLE
Number of units | item					49	49	49
Loan amount								$ 10,000,000
Loan balance					$ 10,000,000
Brookside Associates LLC Residential Apartments Woburn, Massachusetts
MORTGAGE NOTES PAYABLE
Loan amount	$ 6,175,000			$ 6,175,000
Repayment of loan	2,390,000			2,390,000
Closing Costs	$ 136,000			$ 136,000
Interest rate (as a percent)	3.53%			3.53%
Initial Advance | 57 Mill Street
MORTGAGE NOTES PAYABLE
Loan amount		$ 31,000,000
Interest rate (as a percent)		3.586%
Subsequent Advance | 57 Mill Street
MORTGAGE NOTES PAYABLE
Loan amount		$ 4,000,000
Interest rate (as a percent)		3.50%
Maximum | 57 Mill Street
MORTGAGE NOTES PAYABLE
Loan amount		$ 35,000,000
Mortgages payable
MORTGAGE NOTES PAYABLE
Loan balance					$ 373,190,000
Mortgages payable | Minimum
MORTGAGE NOTES PAYABLE
Interest rate (as a percent)					2.97%
Mortgages payable | Maximum
MORTGAGE NOTES PAYABLE
Interest rate (as a percent)					4.95%