REPORTABLE SEGMENTS (UNITED DOMINION REALTY, L.P.) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) segment	Mar. 31, 2020 USD ($) item	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) home	Mar. 31, 2019 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segments
Same store communities			37,910		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				$ 320,093		$ 267,922
Reconciling items:
Property management				(9,203)		(7,703)
Other operating expenses				(4,966)		(5,646)
Real estate depreciation and amortization				(155,476)		(112,468)
General and administrative				(14,978)		(12,467)
Casualty-related (charges)/recoveries, net				(1,251)
Income/(loss) from unconsolidated entities				3,367		49
Interest expense				(39,317)		(33,542)
Net (income)/loss attributable to noncontrolling interests				(6)		(42)
Net income/(loss) attributable to OP unitholders				5,221		24,503
Reportable apartment home segment assets:
Total segment assets	$ 12,817,998	$ 12,817,998	$ 12,817,998	12,817,998	$ 12,817,998		$ 12,602,101
Accumulated depreciation	(4,272,471)	(4,272,471)	(4,272,471)	(4,272,471)	(4,272,471)		(4,131,353)
Total real estate owned, net of accumulated depreciation	8,545,527	8,545,527	8,545,527	8,545,527	8,545,527		8,470,748
Total segment asset - net book value	8,545,527	8,545,527	8,545,527	8,545,527	8,545,527		8,470,748
Reconciling items:
Cash and cash equivalents	980	980	980	980	980	1,043	8,106	$ 185,216
Restricted cash	21,949	21,949	21,949	21,949	21,949	23,111	25,185	23,675
Investment in unconsolidated entities	588,395	588,395	588,395	588,395	588,395		588,262
Operating lease right-of-use assets	203,410	203,410	203,410	203,410	203,410		204,225
Other assets	179,301	179,301	179,301	179,301	179,301		186,296
Total assets	$ 9,691,105	$ 9,691,105	9,691,105	9,691,105	9,691,105		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				102,616		98,633
Other revenue				2,774		3,076
Rental income				105,390		101,709
Reportable apartment home segment NOI				79,823		76,984
Reportable apartment home segment assets:
Total segment assets	$ 3,704,480	$ 3,704,480	3,704,480	3,704,480	3,704,480		3,696,544
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				56,085		54,300
Other revenue				1,573		2,002
Rental income				57,658		56,302
Reportable apartment home segment NOI				40,377		39,179
Reportable apartment home segment assets:
Total segment assets	2,356,688	2,356,688	2,356,688	2,356,688	2,356,688		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				30,942		30,193
Other revenue				501		629
Rental income				31,443		30,822
Reportable apartment home segment NOI				20,614		20,977
Reportable apartment home segment assets:
Total segment assets	1,501,767	1,501,767	1,501,767	1,501,767	1,501,767		1,500,597
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				30,677		29,531
Other revenue				1,429		1,790
Rental income				32,106		31,321
Reportable apartment home segment NOI				22,617		22,010
Reportable apartment home segment assets:
Total segment assets	810,984	810,984	810,984	810,984	810,984		806,830
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				16,626		16,077
Other revenue				590		763
Rental income				17,216		16,840
Reportable apartment home segment NOI				11,056		10,328
Reportable apartment home segment assets:
Total segment assets	603,082	603,082	603,082	603,082	603,082		600,350
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				74,676		30,169
Other revenue				1,604		759
Rental income				76,280		30,928
Reportable apartment home segment NOI				50,978		20,205
Reportable apartment home segment assets:
Total segment assets	3,840,997	3,840,997	$ 3,840,997	3,840,997	$ 3,840,997		3,647,439
Total Communities
Summary of rental income and NOI for UDRs reportable segments and reconciliation of NOI to loss from continuing operations
Lease revenue				311,622		258,903
Other revenue				8,471		9,019
Rental income				320,093		267,922
Reportable apartment home segment NOI				225,465		189,683
United Dominion Realty L.P.
Segments
Same store communities			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				112,165		108,334
Reconciling items:
Property management				(3,225)		(2,979)
Other operating expenses				(3,859)		(2,400)
Real estate depreciation and amortization				(35,300)		(34,654)
General and administrative				(5,308)		(4,661)
Casualty-related (charges)/recoveries, net				2
Income/(loss) from unconsolidated entities				(1,761)		(2,740)
Interest expense				(7,464)		(7,371)
Net (income)/loss attributable to noncontrolling interests				(522)		(388)
Net income/(loss) attributable to OP unitholders				24,118		23,946
Reportable apartment home segment assets:
Total segment assets	3,886,613	3,886,613	$ 3,886,613	3,886,613	$ 3,886,613		3,875,160
Accumulated depreciation	(1,831,814)	(1,831,814)	(1,831,814)	(1,831,814)	(1,831,814)		(1,796,568)
Total real estate owned, net of accumulated depreciation	2,054,799	2,054,799	2,054,799	2,054,799	2,054,799		2,078,592
Total segment asset - net book value	2,054,799	2,054,799	2,054,799	2,054,799	2,054,799		2,078,592
Reconciling items:
Cash and cash equivalents	40	40	40	40	40	59	24	125
Restricted cash	14,121	14,121	14,121	14,121	14,121	14,010	13,998	$ 13,563
Investment in unconsolidated entities	69,617	69,617	69,617	69,617	69,617		76,222
Operating lease right-of-use assets	204,723	204,723	204,723	204,723	204,723		205,668
Other assets	24,042	24,042	24,042	24,042	24,042		24,241
Total assets	$ 2,367,342	$ 2,367,342	2,367,342	2,367,342	2,367,342		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				63,242		60,782
Other revenue				1,779		1,995
Rental income				65,021		62,777
Reportable apartment home segment NOI				49,493		47,797
Reportable apartment home segment assets:
Total segment assets	$ 2,017,542	$ 2,017,542	2,017,542	2,017,542	2,017,542		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				15,164		14,810
Other revenue				431		535
Rental income				15,595		15,345
Reportable apartment home segment NOI				10,796		10,524
Reportable apartment home segment assets:
Total segment assets	671,170	671,170	671,170	671,170	671,170		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				8,229		7,972
Other revenue				101		166
Rental income				8,330		8,138
Reportable apartment home segment NOI				5,909		6,205
Reportable apartment home segment assets:
Total segment assets	408,954	408,954	408,954	408,954	408,954		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				12,987		12,462
Other revenue				644		775
Rental income				13,631		13,237
Reportable apartment home segment NOI				9,550		9,257
Reportable apartment home segment assets:
Total segment assets	354,058	354,058	354,058	354,058	354,058		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				1,830		1,924
Other revenue				71		31
Rental income				1,901		1,955
Reportable apartment home segment NOI				1,391		1,337
Reportable apartment home segment assets:
Total segment assets	144,640	144,640	144,640	144,640	144,640		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				7,550		6,726
Other revenue				137		156
Rental income				7,687		6,882
Reportable apartment home segment NOI				4,416		4,019
Reportable apartment home segment assets:
Total segment assets	$ 290,249	$ 290,249	$ 290,249	290,249	$ 290,249		$ 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				109,002		104,676
Other revenue				3,163		3,658
Rental income				112,165		108,334
Reportable apartment home segment NOI				$ 81,555		$ 79,139