JOINT VENTURES AND PARTNERSHIPS - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial information relating to unconsolidated joint ventures operations
Operating income/(loss)	$ 137,873	$ (5,055)	$ 31,257
Joint Ventures and Partnerships
Financial information relating to unconsolidated joint ventures operations
Total revenues	278,006	287,083	267,116
Property operating expenses	105,349	112,347	103,969
Real estate depreciation and amortization	105,883	115,211	106,805
Gain (loss) on sale of real estate	115,516		71,590
Operating income/(loss)	182,290	59,525	127,932
Interest expense	(80,198)	(87,363)	(78,829)
Gain loss on revaluation of assets and liabilities	483,906
Other income/(loss)	159	148	(439)
Net income /(loss)	586,157	(27,690)	48,664
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net	1,545,106	3,167,984
Cash and cash equivalents	20,190	39,405
Other assets	17,535	29,819
Total assets	1,582,831	3,237,208
Third party debt, net	1,041,663	2,114,420
Accounts payable and accrued liabilities	26,537	59,717
Total liabilities	1,068,200	2,174,137
Total equity	514,631	1,063,071
Unconsolidated Joint Venture Other MetLife
Financial information relating to unconsolidated joint ventures operations
Total revenues	64,273	61,967	48,032
Property operating expenses	22,019	21,998	21,908
Real estate depreciation and amortization	35,001	35,437	32,625
Operating income/(loss)	7,253	4,532	(6,501)
Interest expense	(17,399)	(17,408)	(13,894)
Net income /(loss)	(10,146)	(12,876)	(20,395)
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net	621,335	653,729
Cash and cash equivalents	7,973	8,242
Other assets	5,400	4,904
Total assets	634,708	666,875
Third party debt, net	454,972	454,647
Accounts payable and accrued liabilities	9,757	9,753
Total liabilities	464,729	464,400
Total equity	169,979	202,475
Unconsolidated Joint Venture Vitruvian Park
Financial information relating to unconsolidated joint ventures operations
Total revenues	26,398	26,096	23,025
Property operating expenses	12,541	13,732	11,839
Real estate depreciation and amortization	9,832	9,495	7,169
Operating income/(loss)	4,025	2,869	4,017
Interest expense	(5,948)	(6,051)	(5,030)
Gain loss on revaluation of assets and liabilities	25,711
Net income /(loss)	23,788	(3,182)	(1,013)
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net		315,541
Cash and cash equivalents		8,865
Other assets		2,241
Total assets		326,647
Third party debt, net		162,131
Accounts payable and accrued liabilities		14,968
Total liabilities		177,099
Total equity		149,548
Unconsolidated Joint Venture UDR Met Life I Partnership
Financial information relating to unconsolidated joint ventures operations
Total revenues	9,834	3,187
Property operating expenses	4,533	3,066	93
Real estate depreciation and amortization	5,787	3,392
Gain (loss) on sale of real estate			(17)
Operating income/(loss)	(486)	(3,271)	(110)
Interest expense	(3,070)	(1,872)
Net income /(loss)	(3,556)	(5,143)	(110)
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net	120,055	124,112
Cash and cash equivalents	2,317	698
Other assets	1,053	1,074
Total assets	123,425	125,884
Third party debt, net	70,890	70,833
Accounts payable and accrued liabilities	4,037	1,935
Total liabilities	74,927	72,768
Total equity	48,498	53,116
Unconsolidated Joint Venture UDR MetLife II Partnership
Financial information relating to unconsolidated joint ventures operations
Total revenues	151,226	158,738	156,920
Property operating expenses	54,445	56,403	52,450
Real estate depreciation and amortization	44,077	44,721	45,144
Gain (loss) on sale of real estate			(609)
Operating income/(loss)	52,704	57,614	58,717
Interest expense	(44,825)	(49,118)	(50,603)
Gain loss on revaluation of assets and liabilities	458,195
Net income /(loss)	466,074	8,496	8,114
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net	663,492	1,609,903
Cash and cash equivalents	4,208	11,192
Other assets	9,777	18,670
Total assets	677,477	1,639,765
Third party debt, net	425,303	1,089,231
Accounts payable and accrued liabilities	9,303	21,258
Total liabilities	434,606	1,110,489
Total equity	242,871	529,276
Unconsolidated Joint Venture Three Washington DC
Financial information relating to unconsolidated joint ventures operations
Total revenues	12,217	20,703	20,327
Property operating expenses	4,982	8,318	8,159
Real estate depreciation and amortization	5,746	14,487	14,480
Gain (loss) on sale of real estate	115,516
Operating income/(loss)	117,005	(2,102)	(2,312)
Interest expense	(4,300)	(6,739)	(5,264)
Net income /(loss)	112,705	(8,841)	(7,576)
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net		182,970
Cash and cash equivalents		1,794
Other assets		1,320
Total assets		186,084
Third party debt, net		165,699
Accounts payable and accrued liabilities		1,860
Total liabilities		167,559
Total equity		18,525
Preferred Equity Investment West Coast Development JV
Financial information relating to unconsolidated joint ventures operations
Total revenues	14,058	16,392	18,812
Property operating expenses	6,829	8,830	9,520
Real estate depreciation and amortization	5,440	7,679	7,387
Gain (loss) on sale of real estate			72,216
Operating income/(loss)	1,789	(117)	74,121
Interest expense	(4,656)	(6,175)	(4,038)
Other income/(loss)	159	148	(439)
Net income /(loss)	(2,708)	(6,144)	69,644
Combined summary of balance sheets relating to unconsolidated joint ventures
Total real estate, net	140,224	281,729
Cash and cash equivalents	5,692	8,614
Other assets	1,305	1,610
Total assets	147,221	291,953
Third party debt, net	90,498	171,879
Accounts payable and accrued liabilities	3,440	9,943
Total liabilities	93,938	181,822
Total equity	53,283	110,131
Developer Capital Program excluding West Coast Development Joint Ventures
Financial information relating to unconsolidated joint ventures operations
Total revenues		11,200	6,000	$ 7,800
Expenses		3,500	1,800	$ 9,500
Other income/ loss	$ 26,400
Combined summary of balance sheets relating to unconsolidated joint ventures
Total assets		521,000	248,100
Total liabilities		135,000	22,500
Total equity		$ 386,000	$ 225,600