DERIVATIVES AND HEDGING ACTIVITY - Fair Value (UNITED DOMINION REALTY, L.P.) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Effect of derivative instruments on the Consolidated Statements of Operations
Unrealized holding gain/(loss)	$ (30)	$ (659)	$ (3,241)	$ (7,181)
United Dominion Realty L.P.
Effect of derivative instruments on the Consolidated Statements of Operations
Unrealized holding gain/(loss)	(6)		(49)
Interest rate caps | United Dominion Realty L.P. | Interest expense | Cash Flow Hedging
Effect of derivative instruments on the Consolidated Statements of Operations
Unrealized holding gain/(loss)	(6)		(49)
Interest rate contracts | Interest expense | Cash Flow Hedging
Effect of derivative instruments on the Consolidated Statements of Operations
Unrealized holding gain/(loss)	(30)	(659)	(3,241)	(7,181)
Gain/(Loss) reclassified from Accumulated OCI in Interest Expense	$ (1,585)	$ 624	$ (3,234)	$ 2,504