LEASES (UNITED DOMINION REALTY, L.P.) - Lessee Future Minimum Payments - (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) community lease	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Lessee operating leases
Number of communities subject to ground leases | community	6
Number of leases | lease	6
Operating leases existence of option to extend	true
Operating lease right-of-use assets	$ 201,754		$ 204,225
Increase in operating lease right-to-use assets		$ 42,143
Weighted average remaining lease term	44 years 1 month 6 days
Weighted average discount rate	5.00%
Number of real estate properties | community	148
Future minimum lease payments
Operating lease liabilities	$ 196,346		198,558
Ground Leases
Future minimum lease payments
2020	3,109
2021	12,442
2022	12,442
2023	12,442
2024	12,442
Thereafter	455,221
Total future minimum lease payments (undiscounted)	508,098
Difference between future undiscounted cash flows and discounted cash flows	(311,752)
Operating lease liabilities	$ 196,346
United Dominion Realty L.P.
Lessee operating leases
Number of communities subject to ground leases | community	6
Operating leases existence of option to extend	true
Operating lease right-of-use assets	$ 203,163		205,668
Increase in operating lease right-to-use assets		$ 42,143
Weighted average remaining lease term	43 years 10 months 24 days
Weighted average discount rate	5.00%
Number of real estate properties | community	52
Future minimum lease payments
2020	$ 3,150
2021	12,606
2022	12,609
2023	12,613
2024	12,616
Thereafter	456,122
Total future minimum lease payments (undiscounted)	509,716
Difference between future undiscounted cash flows and discounted cash flows	(311,945)
Operating lease liabilities	197,771		$ 200,001
United Dominion Realty L.P. | Ground Leases
Future minimum lease payments
2020	3,109
2021	12,442
2022	12,442
2023	12,442
2024	12,442
Thereafter	455,221
Total future minimum lease payments (undiscounted)	508,098
Difference between future undiscounted cash flows and discounted cash flows	(311,752)
Operating lease liabilities	196,346
United Dominion Realty L.P. | Equipment Leases
Future minimum lease payments
2020	41
2021	164
2022	167
2023	171
2024	174
Thereafter	901
Total future minimum lease payments (undiscounted)	1,618
Difference between future undiscounted cash flows and discounted cash flows	(193)
Operating lease liabilities	$ 1,425