LEASES (UNITED DOMINION REALTY, L.P.) - Lessee Future Minimum Payments - (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) lease community	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019
Lessee operating leases
Number of communities subject to ground leases | community	6
Operating leases existence of option to extend	true
Lease classification per practical expedient	true
Operating lease right-of-use assets	$ 204,225
Increase in operating lease right-to-use assets	$ 111,055
Number of ground leases that became fixed | lease	2
Weighted average remaining lease term				44 years 8 months 12 days
Weighted average discount rate	5.00%
Number of real estate properties | community	148
Future minimum lease payments
Operating lease liabilities	$ 198,558
LAND
Future minimum lease payments
2020	12,442
2021	12,442
2022	12,442
2023	12,442
2024	12,442
Thereafter	455,221
Total future minimum lease payments (undiscounted)	517,431
Difference between future undiscounted cash flows and discounted cash flows	(318,873)
Operating lease liabilities	$ 198,558
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	$ 205,668
Increase in operating lease right-to-use assets	$ 112,498
Weighted average remaining lease term	44 years 4 months 24 days
Weighted average discount rate	5.00%
Number of real estate properties | community	52
Future minimum lease payments
2020	$ 12,594
2021	12,598
2022	12,601
2023	12,605
2024	12,608
Thereafter	456,090
Total future minimum lease payments (undiscounted)	519,096
Difference between future undiscounted cash flows and discounted cash flows	(319,095)
Operating lease liabilities	200,001
United Dominion Realty L.P. | LAND
Lessee operating leases
Increase in operating lease right-to-use assets	111,100
2019		$ 4,900
2020		4,900
2021		4,900
2022		4,900
2023		4,900
Thereafter		313,900
Future minimum lease payments
2020	12,442
2021	12,442
2022	12,442
2023	12,442
2024	12,442
Thereafter	455,221
Total future minimum lease payments (undiscounted)	517,431
Difference between future undiscounted cash flows and discounted cash flows	(318,873)
Operating lease liabilities	$ 198,558
United Dominion Realty L.P. | LAND | Other operating expense
Lessee operating leases
Rent expense		$ 7,300	$ 6,200
United Dominion Realty L.P. | Equipment Leases
Lessee operating leases
Number of leases | lease	6
Increase in operating lease right-to-use assets	$ 1,400
Future minimum lease payments
2020	152
2021	156
2022	159
2023	163
2024	166
Thereafter	869
Total future minimum lease payments (undiscounted)	1,665
Difference between future undiscounted cash flows and discounted cash flows	(222)
Operating lease liabilities	$ 1,443
United Dominion Realty L.P. | Retail and Commercial Spaces
Lessee operating leases
Lease classification per practical expedient	true