REPORTABLE SEGMENTS (UNITED DOMINION REALTY, L.P.) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) home	Jun. 30, 2019 USD ($) home	Jun. 30, 2020 USD ($) segment home	Jun. 30, 2019 USD ($) home	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segments
Same store communities | home	39,020	39,020	37,910	37,910
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	$ 305,982	$ 278,463	$ 626,075	$ 546,385
Reconciling items:
Property management	(8,797)	(8,006)	(18,000)	(15,709)
Other operating expenses	(6,100)	(2,735)	(11,066)	(8,381)
Real estate depreciation and amortization	(155,056)	(117,934)	(310,532)	(230,402)
General and administrative	(10,971)	(12,338)	(25,949)	(24,805)
Casualty-related (charges)/recoveries, net	(102)	(246)	(1,353)	(246)
Income/(loss) from unconsolidated entities	8,021	6,625	11,388	6,674
Interest expense	(38,597)	(34,417)	(77,914)	(67,959)
Net (income)/loss attributable to noncontrolling interests	(34)	(47)	(40)	(89)
Net income/(loss) attributable to OP unitholders	57,771	35,619	62,992	60,122
Reportable apartment home segment assets:
Total segment assets	12,775,639		12,775,639		$ 12,602,101
Accumulated depreciation	(4,372,524)		(4,372,524)		(4,131,353)
Total real estate owned, net of accumulated depreciation	8,403,115		8,403,115		8,470,748
Total segment asset - net book value	8,403,115		8,403,115		8,470,748
Reconciling items:
Cash and cash equivalents	833	981	833	981	8,106	$ 185,216
Restricted cash	22,043	23,042	22,043	23,042	25,185	23,675
Investment in unconsolidated entities	598,058		598,058		588,262
Operating lease right-of-use assets	202,586		202,586		204,225
Other assets	181,880		181,880		186,296
Total assets	9,564,471		$ 9,564,471		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	101,551	104,196	$ 200,750	199,495
Other revenue	2,842	3,047	5,543	6,045
Rental income	104,393	107,243	206,293	205,540
Reportable apartment home segment NOI	78,533	81,724	155,876	156,324
Reportable apartment home segment assets:
Total segment assets	3,889,925		3,889,925		3,874,027
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	54,803	54,905	110,889	109,205
Other revenue	1,429	1,727	3,001	3,729
Rental income	56,232	56,632	113,890	112,934
Reportable apartment home segment NOI	39,102	40,045	79,479	79,225
Reportable apartment home segment assets:
Total segment assets	2,363,674		2,363,674		2,350,341
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	29,717	32,225	58,774	60,490
Other revenue	761	789	1,235	1,396
Rental income	30,478	33,014	60,009	61,886
Reportable apartment home segment NOI	19,629	22,943	38,734	42,453
Reportable apartment home segment assets:
Total segment assets	1,635,118		1,635,118		1,632,980
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	32,715	31,844	61,433	59,565
Other revenue	1,226	1,719	2,593	3,431
Rental income	33,941	33,563	64,026	62,996
Reportable apartment home segment NOI	22,801	23,322	44,257	44,169
Reportable apartment home segment assets:
Total segment assets	913,257		913,257		903,878
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	16,391	16,294	33,017	32,370
Other revenue	515	678	1,105	1,443
Rental income	16,906	16,972	34,122	33,813
Reportable apartment home segment NOI	10,467	10,444	21,523	20,772
Reportable apartment home segment assets:
Total segment assets	605,801		605,801		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	62,404	30,523	144,340	67,763
Other revenue	1,628	516	3,395	1,453
Rental income	64,032	31,039	147,735	69,216
Reportable apartment home segment NOI	41,721	21,257	97,849	46,475
Reportable apartment home segment assets:
Total segment assets	3,367,864		3,367,864		3,240,526
Total Communities
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue	297,581	269,987	609,203	528,888
Other revenue	8,401	8,476	16,872	17,497
Rental income	305,982	278,463	626,075	546,385
Reportable apartment home segment NOI	$ 212,253	$ 199,735	$ 437,718	389,418
United Dominion Realty L.P.
Segments
Same store communities | home	15,941	15,941	15,941
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	$ 105,201	$ 110,350	$ 217,366	218,684
Reconciling items:
Property management	(3,024)	(3,308)	(6,249)	(6,287)
Other operating expenses	(3,837)	(2,422)	(7,696)	(4,822)
Real estate depreciation and amortization	(35,430)	(34,921)	(70,730)	(69,575)
General and administrative	(3,951)	(4,151)	(9,259)	(8,812)
Casualty-related (charges)/recoveries, net	(190)	81	(188)	81
Income/(loss) from unconsolidated entities	(1,661)	(1,794)	(3,422)	(4,534)
Interest expense	(7,170)	(7,355)	(14,634)	(14,726)
Net (income)/loss attributable to noncontrolling interests	(488)	(416)	(1,010)	(804)
Net income/(loss) attributable to OP unitholders	19,373	27,394	43,491	51,340
Reportable apartment home segment assets:
Total segment assets	3,898,710		3,898,710		3,875,160
Accumulated depreciation	(1,867,071)		(1,867,071)		(1,796,568)
Total real estate owned, net of accumulated depreciation	2,031,639		2,031,639		2,078,592
Total segment asset - net book value	2,031,639		2,031,639		2,078,592
Reconciling items:
Cash and cash equivalents	38	50	38	50	24	125
Restricted cash	14,535	14,124	14,535	14,124	13,998	$ 13,563
Investment in unconsolidated entities	63,111		63,111		76,222
Operating lease right-of-use assets	203,958		203,958		205,668
Other assets	25,643		25,643		24,241
Total assets	2,338,924		$ 2,338,924		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	60,897	62,298	$ 124,140	123,080
Other revenue	1,603	1,852	3,379	3,846
Rental income	62,500	64,150	127,519	126,926
Reportable apartment home segment NOI	47,645	49,348	97,138	97,145
Reportable apartment home segment assets:
Total segment assets	2,023,401		2,023,401		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	14,656	14,881	29,820	29,691
Other revenue	376	429	807	964
Rental income	15,032	15,310	30,627	30,655
Reportable apartment home segment NOI	10,231	10,555	21,027	21,079
Reportable apartment home segment assets:
Total segment assets	673,232		673,232		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	7,329	7,966	15,558	15,938
Other revenue	94	168	196	334
Rental income	7,423	8,134	15,754	16,272
Reportable apartment home segment NOI	5,061	6,260	10,970	12,466
Reportable apartment home segment assets:
Total segment assets	409,242		409,242		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	12,949	12,686	25,936	25,148
Other revenue	549	727	1,193	1,502
Rental income	13,498	13,413	27,129	26,650
Reportable apartment home segment NOI	9,021	9,356	18,572	18,613
Reportable apartment home segment assets:
Total segment assets	356,006		356,006		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	1,834	1,950	3,664	3,875
Other revenue	35	91	106	122
Rental income	1,869	2,041	3,770	3,997
Reportable apartment home segment NOI	1,334	1,484	2,725	2,820
Reportable apartment home segment assets:
Total segment assets	144,751		144,751		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	4,672	7,326	12,222	14,052
Other revenue	207	(24)	345	132
Rental income	4,879	7,302	12,567	14,184
Reportable apartment home segment NOI	1,832	4,677	6,247	8,696
Reportable apartment home segment assets:
Total segment assets	292,078		292,078		$ 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	102,337	107,107	211,340	211,784
Other revenue	2,864	3,243	6,026	6,900
Rental income	105,201	110,350	217,366	218,684
Reportable apartment home segment NOI	$ 75,124	$ 81,680	$ 156,679	$ 160,819