DEBT - Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) apartmentProperty	Dec. 31, 2021 USD ($) apartmentProperty loan	Jan. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Number of real estate properties, unencumbered by mortgages | apartmentProperty		48
Maximum borrowing capacity		$ 6,000,000
Revolving lines of credit		$ 76,000,000		$ 152,871,000
Annual Weighted Average Interest Rates		3.26%		3.62%
Carrying principal value		$ 859,784,000		$ 721,316,000
Number of real estate communities, serving as collateral for mortgage loans | apartmentProperty		15
Domestic Line of Credit | Base Rate | Minimum
Line of Credit Facility [Line Items]
Interest rate spread (as a percent)		0.25%
Domestic Line of Credit | Base Rate | Maximum
Line of Credit Facility [Line Items]
Interest rate spread (as a percent)		0.80%
Domestic Line of Credit | London Interbank Offered Rate (LIBOR) | Minimum
Line of Credit Facility [Line Items]
Interest rate spread (as a percent)		1.25%
Domestic Line of Credit | London Interbank Offered Rate (LIBOR) | Maximum
Line of Credit Facility [Line Items]
Interest rate spread (as a percent)		1.80%
BMO Line of Credit | Lines of credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 250,000,000		250,000,000
Revolving lines of credit				152,900,000
Accordion option	$ 400,000,000
BMO Line of Credit | Domestic Line of Credit
Line of Credit Facility [Line Items]
Remaining borrowing capacity		173,500,000
Revolving lines of credit		$ 76,000,000
Operating line of credit, interest rate		2.74%
Mortgages payable - Fannie Mae credit facility
Line of Credit Facility [Line Items]
Number of loans in default | loan		0
Mortgages payable - Fannie Mae credit facility | Minimum
Line of Credit Facility [Line Items]
Mortgage loans, interest rate		3.47%
Mortgages payable - Fannie Mae credit facility | Maximum
Line of Credit Facility [Line Items]
Mortgage loans, interest rate		4.31%
Mortgages payable - Fannie Mae credit facility | Fannie Mae Credit Facility Agreement
Line of Credit Facility [Line Items]
Original principal balance	$ 198,900,000
Annual Weighted Average Interest Rates	2.78%
Carrying principal value		$ 198,850,000		$ 0
Mortgages payable - Fannie Mae credit facility | Fannie Mae Credit Facility Agreement | Interest Only Payment Date One
Line of Credit Facility [Line Items]
Term	7 years
Mortgages payable - Fannie Mae credit facility | Fannie Mae Credit Facility Agreement | Interest Only Payment Date Two
Line of Credit Facility [Line Items]
Term	10 years
Mortgages payable - Fannie Mae credit facility | Fannie Mae Credit Facility Agreement | Interest Only Payment Date Three
Line of Credit Facility [Line Items]
Term	12 years
Mortgages payable - Fannie Mae credit facility | Fannie Mae Credit Facility Agreement | 2021 Acquisitions
Line of Credit Facility [Line Items]
Number of apartment units | apartmentProperty	16
Unsecured debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity		225,000,000
Remaining borrowing capacity		25,000,000
Unsecured debt | Term Loan Maturing 2024
Line of Credit Facility [Line Items]
Original principal balance		70,000,000
Unsecured debt | Term Loan Maturing 2025
Line of Credit Facility [Line Items]
Original principal balance		75,000,000
Unsecured debt | Private Shelf Agreement
Line of Credit Facility [Line Items]
Carrying principal value			$ 200,000,000
Unsecured debt | Note Purchase Agreement
Line of Credit Facility [Line Items]
Original principal balance		$ 125,000,000