REPORTABLE SEGMENTS (UNITED DOMINION REALTY, L.P.) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) home	Sep. 30, 2019 USD ($) home	Sep. 30, 2020 USD ($) home segment	Sep. 30, 2019 USD ($) home	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segments
Same store communities | home	40,258	40,258	37,607	37,607
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	$ 308,845	$ 289,008	$ 934,920	$ 835,393
Reconciling items:
Property management	(8,879)	(8,309)	(26,879)	(24,018)
Other operating expenses	(5,543)	(2,751)	(16,609)	(11,132)
Real estate depreciation and amortization	(151,949)	(127,391)	(462,481)	(357,793)
General and administrative	(11,958)	(12,197)	(37,907)	(37,002)
Casualty-related (charges)/recoveries, net		1,088	(1,353)	842
Gain/(loss) on sales of real estate owned			61,303	5,282
Income/(loss) from unconsolidated entities	2,940	12,713	14,328	19,387
Interest expense	(62,268)	(42,523)	(140,182)	(110,482)
Net (income)/loss attributable to noncontrolling interests	(31)	(56)	(71)	(145)
Net income/(loss) attributable to OP unitholders	(25,258)	27,204	37,734	87,326
Reportable apartment home segment assets:
Total segment assets	12,861,020		12,861,020		$ 12,602,101
Accumulated depreciation	(4,512,771)		(4,512,771)		(4,131,353)
Total real estate owned, net of accumulated depreciation	8,348,249		8,348,249		8,470,748
Total segment asset - net book value	8,348,249		8,348,249		8,470,748
Reconciling items:
Cash and cash equivalents	927	1,895	927	1,895	8,106	$ 185,216
Restricted cash	23,273	21,646	23,273	21,646	25,185	23,675
Investment in unconsolidated entities	646,355		646,355		588,262
Operating lease right-of-use assets	201,754		201,754		204,225
Other assets	173,834		173,834		186,296
Total assets	9,551,388		$ 9,551,388		9,636,472
Reportable Segments
Number of reportable segments | segment			2
Condition for Community considered to have stabilized occupancy			90%
Time to maintain percent occupancy to be considered a community			3 months
Practical expedient, single lease component			true
Same Store Communities West Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	102,609	110,860	$ 294,355	301,291
Other revenue	3,418	3,510	8,788	9,313
Rental income	106,027	114,370	303,143	310,604
Reportable apartment home segment NOI	77,361	86,159	226,538	235,925
Reportable apartment home segment assets:
Total segment assets	4,253,098		4,253,098		4,228,359
Same Store Communities Mid-Atlantic Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	52,411	53,019	159,015	157,914
Other revenue	1,831	2,012	4,725	5,611
Rental income	54,242	55,031	163,740	163,525
Reportable apartment home segment NOI	37,258	38,483	113,913	114,892
Reportable apartment home segment assets:
Total segment assets	2,244,315		2,244,315		2,222,405
Same Store Communities Northeast Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	32,705	40,558	83,314	91,203
Other revenue	1,102	1,008	2,051	2,200
Rental income	33,807	41,566	85,365	93,403
Reportable apartment home segment NOI	18,981	27,962	52,020	63,012
Reportable apartment home segment assets:
Total segment assets	2,079,168		2,079,168		2,073,052
Same Store Communities Southeast Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	34,207	33,283	92,555	89,859
Other revenue	1,436	1,782	3,949	5,115
Rental income	35,643	35,065	96,504	94,974
Reportable apartment home segment NOI	23,714	24,033	66,029	66,301
Reportable apartment home segment assets:
Total segment assets	967,465		967,465		953,029
Same Store Communities Southwest Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	16,418	16,229	49,435	48,600
Other revenue	710	749	1,815	2,191
Rental income	17,128	16,978	51,250	50,791
Reportable apartment home segment NOI	10,496	10,449	32,019	31,221
Reportable apartment home segment assets:
Total segment assets	607,326		607,326		600,349
Non-Mature communities/Other
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	58,848	25,292	227,725	119,262
Other revenue	3,150	706	7,193	2,834
Rental income	61,998	25,998	234,918	122,096
Reportable apartment home segment NOI	43,322	16,563	158,331	81,716
Reportable apartment home segment assets:
Total segment assets	2,709,648		2,709,648		2,524,907
Total Communities
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	297,198	279,241	906,399	808,129
Other revenue	11,647	9,767	28,521	27,264
Rental income	308,845	289,008	934,920	835,393
Reportable apartment home segment NOI	$ 211,132	$ 203,649	$ 648,850	593,067
United Dominion Realty L.P.
Segments
Same store communities | home	15,609	15,609	15,609
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	$ 107,694	$ 111,700	$ 325,060	330,384
Reconciling items:
Property management	(3,095)	(3,211)	(9,345)	(9,498)
Other operating expenses	(4,110)	(2,301)	(11,806)	(7,123)
Real estate depreciation and amortization	(35,335)	(35,155)	(106,065)	(104,730)
General and administrative	(4,323)	(4,066)	(13,582)	(12,878)
Casualty-related (charges)/recoveries, net		1,088	(188)	1,169
Income/(loss) from unconsolidated entities	(1,562)	(2,383)	(4,984)	(6,917)
Interest expense	(7,267)	(7,521)	(21,901)	(22,247)
Net (income)/loss attributable to noncontrolling interests	(435)	(448)	(1,445)	(1,252)
Net income/(loss) attributable to OP unitholders	18,622	26,835	62,113	78,175
Reportable apartment home segment assets:
Total segment assets	3,912,678		3,912,678		3,875,160
Accumulated depreciation	(1,902,081)		(1,902,081)		(1,796,568)
Total real estate owned, net of accumulated depreciation	2,010,597		2,010,597		2,078,592
Total segment asset - net book value	2,010,597		2,010,597		2,078,592
Reconciling items:
Cash and cash equivalents	81	40	81	40	24	125
Restricted cash	15,455	14,637	15,455	14,637	13,998	$ 13,563
Investment in unconsolidated entities	56,705		56,705		76,222
Operating lease right-of-use assets	203,163		203,163		205,668
Other assets	26,909		26,909		24,241
Total assets	2,312,910		$ 2,312,910		2,398,745
Reportable Segments
Number of reportable segments | segment			2
Condition for Community considered to have stabilized occupancy			90%
Time to maintain percent occupancy to be considered a community			3 months
Practical expedient, single lease component			true
United Dominion Realty L.P. | Same Store Communities West Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	58,848	62,760	$ 182,988	185,840
Other revenue	1,996	2,063	5,375	5,908
Rental income	60,844	64,823	188,363	191,748
Reportable apartment home segment NOI	45,011	49,528	142,148	146,673
Reportable apartment home segment assets:
Total segment assets	2,030,008		2,030,008		2,011,495
United Dominion Realty L.P. | Same Store Communities Mid-Atlantic Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	12,482	12,805	38,018	38,187
Other revenue	482	476	1,180	1,308
Rental income	12,964	13,281	39,198	39,495
Reportable apartment home segment NOI	8,781	9,244	26,984	27,508
Reportable apartment home segment assets:
Total segment assets	547,810		547,810		669,417
United Dominion Realty L.P. | Same Store Communities Northeast Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	6,013	8,098	21,572	24,036
Other revenue	168	187	364	521
Rental income	6,181	8,285	21,936	24,557
Reportable apartment home segment NOI	3,055	5,736	14,024	18,201
Reportable apartment home segment assets:
Total segment assets	409,782		409,782		408,703
United Dominion Realty L.P. | Same Store Communities Southeast Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	13,189	12,808	39,124	37,957
Other revenue	663	799	1,856	2,301
Rental income	13,852	13,607	40,980	40,258
Reportable apartment home segment NOI	9,355	9,371	27,926	27,984
Reportable apartment home segment assets:
Total segment assets	358,178		358,178		352,790
United Dominion Realty L.P. | Same Store Communities Southwest Region
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	1,762	1,901	5,426	5,776
Other revenue	65	87	171	209
Rental income	1,827	1,988	5,597	5,985
Reportable apartment home segment NOI	1,266	1,405	3,991	4,226
Reportable apartment home segment assets:
Total segment assets	144,858		144,858		144,210
United Dominion Realty L.P. | Non-Mature communities/Other
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	10,316	9,563	26,822	27,924
Other revenue	1,710	153	2,164	417
Rental income	12,026	9,716	28,986	28,341
Reportable apartment home segment NOI	7,281	5,548	16,356	17,059
Reportable apartment home segment assets:
Total segment assets	422,042		422,042		$ 288,545
United Dominion Realty L.P. | Total Communities
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	102,610	107,935	313,950	319,720
Other revenue	5,084	3,765	11,110	10,664
Rental income	107,694	111,700	325,060	330,384
Reportable apartment home segment NOI	$ 74,749	$ 80,832	$ 231,429	$ 241,651