FAIR VALUE OF DERIVATIVES AND FINANCIAL INSTRUMENTS (UNITED DOMINION REALTY, L.P.) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Debt instruments - fair value
Transfer between the levels	$ 0
Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Total assets	156,998	$ 153,656
Total liabilities	4,945,692	4,731,595
Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Total assets	168,124	160,203
Total liabilities	5,333,919	4,853,337
Interest rate contracts \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Interest rate contracts \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Level 2 \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Total assets	2	6
Total liabilities	1,236	142
Level 2 \| Interest rate contracts \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	2	6
Level 3 \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Total assets	168,122	160,197
Total liabilities	5,332,683	4,853,195
United Dominion Realty L.P. \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Total assets	6
Total liabilities	765,103	736,733
United Dominion Realty L.P. \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Total assets	6
Total liabilities	771,510	736,209
United Dominion Realty L.P. \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Fair value		72,500
United Dominion Realty L.P. \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value		71,976
United Dominion Realty L.P. \| Interest rate contracts \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	6
United Dominion Realty L.P. \| Interest rate contracts \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	6
United Dominion Realty L.P. \| Notes payable due to the General Partner \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Fair value	665,603	637,233
United Dominion Realty L.P. \| Notes payable due to the General Partner \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	665,603	637,233
United Dominion Realty L.P. \| Level 2 \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Total assets	6
United Dominion Realty L.P. \| Level 2 \| Interest rate contracts \| Fair Value, Measurements, Recurring
Debt instruments - fair value
Derivative Asset Designated as Hedging Instrument, Fair Value	6
United Dominion Realty L.P. \| Level 3 \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Total liabilities	771,510	736,209
United Dominion Realty L.P. \| Level 3 \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value		71,976
United Dominion Realty L.P. \| Level 3 \| Notes payable due to the General Partner \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	665,603	637,233
Fixed Rate Debt \| United Dominion Realty L.P. \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Fair value	72,500
Fixed Rate Debt \| United Dominion Realty L.P. \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	78,907
Fixed Rate Debt \| United Dominion Realty L.P. \| Level 3 \| Mortgages Note Payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	78,907
Variable Rate Debt \| United Dominion Realty L.P. \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring \| Carrying Amount
Debt instruments - fair value
Fair value	27,000	27,000
Variable Rate Debt \| United Dominion Realty L.P. \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	27,000	27,000
Variable Rate Debt \| United Dominion Realty L.P. \| Level 3 \| Tax-exempt secured notes payable \| Fair Value, Measurements, Recurring \| Fair Value
Debt instruments - fair value
Fair value	$ 27,000	$ 27,000

Fair value transfers between the levels.

No definition available.

Name:	udr_TransferBetweenLevels
Namespace Prefix:	udr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226006-175313

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41678-113959

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=udr_UnitedDominionRealityLPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=udr_NotesPayableRelatedPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=udr_FixedRateDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=udr_VariableRateDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=udr_TaxExemptNotesPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: