REPORTABLE SEGMENTS (UNITED DOMINION REALTY, L.P.) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment home	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segments
Same store communities | home									37,959
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	$ 302,745	$ 289,008	$ 278,463	$ 267,922	$ 264,732	$ 263,256	$ 256,634	$ 250,483	$ 1,138,138	$ 1,035,105	$ 984,309
Reconciling items:
Property management									(32,721)	(28,465)	(27,068)
Other operating expenses									(13,932)	(12,100)	(9,060)
Real estate depreciation and amortization									(501,257)	(429,006)	(430,054)
General and administrative									(51,533)	(46,983)	(48,566)
Casualty-related (charges)/recoveries, net									(474)	(2,121)	(4,335)
Gain/(loss) on sale of real estate owned									5,282	136,197	43,404
Income/(loss) from unconsolidated entities									137,873	(5,055)	31,257
Interest expense									(170,917)	(134,168)	(128,711)
Net (income)/loss attributable to noncontrolling interests									(188)	(221)	(164)
Net income/(loss) attributable to OP unitholders									184,965	203,106	121,558
Reportable apartment home segment assets:
Total segment assets	12,602,101				10,196,159				12,602,101	10,196,159
Accumulated depreciation	(4,131,353)				(3,654,160)				(4,131,353)	(3,654,160)
Total real estate owned, net of accumulated depreciation	8,470,748				6,541,999				8,470,748	6,541,999
Total segment asset - net book value	8,470,748				6,541,999				8,470,748	6,541,999
Reconciling items:
Cash and cash equivalents	8,106				185,216				8,106	185,216	2,038	$ 2,112
Restricted cash	25,185				23,675				25,185	23,675	19,792	19,994
Investment in unconsolidated entities	588,262				780,869				588,262	780,869
Operating lease right-of-use assets	204,225								204,225
Other assets	186,296				137,710				186,296	137,710
Total assets	9,636,472				7,711,728				$ 9,636,472	7,711,728
Reportable Segments
Number of reportable segments | segment									2
Time to maintain percent occupancy to be considered a community									3 months
Practical expedient, single lease component									true
Capital expenditures and development									$ 195,981	214,898	369,029
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									392,456	377,259	361,277
Other revenue									31,045	28,493	27,493
Rental income									423,501	405,752	388,770
Reportable apartment home segment NOI									321,890	306,307	291,265
Reportable apartment home segment assets:
Total segment assets	3,810,672				3,763,366				3,810,672	3,763,366
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									210,391	204,733	199,206
Other revenue									17,049	15,717	14,951
Rental income									227,440	220,450	214,157
Reportable apartment home segment NOI									159,665	153,670	150,126
Reportable apartment home segment assets:
Total segment assets	2,350,341				2,317,369				2,350,341	2,317,369
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									113,175	109,189	104,106
Other revenue									13,557	13,045	12,361
Rental income									126,732	122,234	116,467
Reportable apartment home segment NOI									88,467	85,220	80,726
Reportable apartment home segment assets:
Total segment assets	806,830				779,310				806,830	779,310
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									119,910	117,572	115,713
Other revenue									4,858	4,590	4,307
Rental income									124,768	122,162	120,020
Reportable apartment home segment NOI									83,832	84,059	83,569
Reportable apartment home segment assets:
Total segment assets	1,500,597				1,491,994				1,500,597	1,491,994
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									162,969	94,176	77,524
Other revenue									12,900	12,080	10,270
Rental income									175,869	106,256	87,794
Reportable apartment home segment NOI									117,860	68,353	58,378
Reportable apartment home segment assets:
Total segment assets	3,677,521				1,396,815				3,677,521	1,396,815
Total Communities
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue									1,053,417	955,899	909,775
Other revenue									84,721	79,206	74,534
Rental income									1,138,138	1,035,105	984,309
Reportable apartment home segment NOI									$ 808,303	732,115	698,503
United Dominion Realty L.P.
Segments
Same store communities | home									15,723
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Rental income	111,389	111,700	110,350	108,334	108,523	109,539	107,266	106,592	$ 441,773	431,920	419,377
Reconciling items:
Property management									(12,701)	(11,878)	(11,533)
Other operating expenses									(9,488)	(8,864)	(6,833)
Real estate depreciation and amortization									(139,975)	(143,481)	(152,473)
General and administrative									(18,014)	(16,889)	(17,875)
Casualty-related (charges)/recoveries, net									(853)	(951)	(1,922)
Gain/(loss) on sale of real estate owned										75,507	41,272
Income/(loss) from unconsolidated entities									(8,313)	43,496	(19,256)
Interest expense									(29,667)	(22,835)	(30,366)
Net (income)/loss attributable to noncontrolling interests									(1,832)	(1,722)	(1,548)
Net income/(loss) attributable to OP unitholders	23,988	$ 26,835	$ 27,394	$ 23,946	85,880	$ 27,695	$ 24,761	$ 91,427	102,163	229,763	106,307
Reportable apartment home segment assets:
Total segment assets	3,875,160				3,811,985				3,875,160	3,811,985
Accumulated depreciation	(1,796,568)				(1,658,161)				(1,796,568)	(1,658,161)
Total real estate owned, net of accumulated depreciation	2,078,592				2,153,824				2,078,592	2,153,824
Total segment asset - net book value	2,078,592				2,153,824				2,078,592	2,153,824
Reconciling items:
Cash and cash equivalents	24				125				24	125	293	756
Restricted cash	13,998				13,563				13,998	13,563	12,579	$ 11,694
Investment in unconsolidated entities	76,222				103,026				76,222	103,026
Operating lease right-of-use assets	205,668								205,668
Other assets	24,241				34,052				24,241	34,052
Total assets	2,398,745				2,304,590				$ 2,398,745	2,304,590
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
Capital expenditures and development									$ 63,175	44,353	45,211
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									248,474	238,886	228,027
Other revenue									7,873	7,161	6,996
Rental income									256,347	246,047	235,023
Reportable apartment home segment NOI									196,302	187,664	177,228
Reportable apartment home segment assets:
Total segment assets	2,011,495				1,981,007				2,011,495	1,981,007
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									59,530	58,624	57,275
Other revenue									1,975	1,765	1,730
Rental income									61,505	60,389	59,005
Reportable apartment home segment NOI									42,413	41,642	40,292
Reportable apartment home segment assets:
Total segment assets	669,417				663,083				669,417	663,083
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									50,795	49,132	46,946
Other revenue									2,925	2,764	2,640
Rental income									53,720	51,896	49,586
Reportable apartment home segment NOI									37,340	35,948	34,182
Reportable apartment home segment assets:
Total segment assets	352,790				340,722				352,790	340,722
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									32,224	31,693	31,387
Other revenue									646	622	589
Rental income									32,870	32,315	31,976
Reportable apartment home segment NOI									24,103	24,578	24,510
Reportable apartment home segment assets:
Total segment assets	408,703				406,149				408,703	406,149
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									36,735	40,547	43,127
Other revenue									596	726	660
Rental income									37,331	41,273	43,787
Reportable apartment home segment NOI									22,848	27,548	30,629
Reportable apartment home segment assets:
Total segment assets	$ 432,755				$ 421,024				432,755	421,024
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									427,758	418,882	406,762
Other revenue									14,015	13,038	12,615
Rental income									441,773	431,920	419,377
Reportable apartment home segment NOI									$ 323,006	$ 317,380	$ 306,841