FAIR VALUE OF DERIVATIVES AND FINANCIAL INSTRUMENTS (UNITED DOMINION REALTY, L.P.) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt instruments - fair value
Transfer between the levels	$ 0
Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Total liabilities	3,827,696	$ 3,688,070
Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Total liabilities	3,773,975	3,746,572
Level 3 | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Total liabilities	3,773,975	3,746,572
United Dominion Reality L.P. | Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Total liabilities	160,205	160,205
United Dominion Reality L.P. | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Total liabilities	162,030	164,150
United Dominion Reality L.P. | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Fair value		27,000
United Dominion Reality L.P. | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		27,000
United Dominion Reality L.P. | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Fair value		133,205
United Dominion Reality L.P. | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		137,150
United Dominion Reality L.P. | Level 3 | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Total liabilities	162,030	164,150
United Dominion Reality L.P. | Level 3 | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		27,000
United Dominion Reality L.P. | Level 3 | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		137,150
Fixed Rate Debt | United Dominion Reality L.P. | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Fair value	133,205
Fixed Rate Debt | United Dominion Reality L.P. | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	135,030
Fixed Rate Debt | United Dominion Reality L.P. | Level 3 | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	135,030
Variable Rate Debt | United Dominion Reality L.P. | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Carrying Amount
Debt instruments - fair value
Fair value	27,000
Variable Rate Debt | United Dominion Reality L.P. | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	27,000
Variable Rate Debt | United Dominion Reality L.P. | Level 3 | Tax-exempt secured notes payable | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	27,000
Fannie Mae credit facilities | Fixed Rate Debt | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	288,323	292,227
Fannie Mae credit facilities | Fixed Rate Debt | Level 3 | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value	$ 288,323	292,227
Fannie Mae credit facilities | Variable Rate Debt | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		29,034
Fannie Mae credit facilities | Variable Rate Debt | Level 3 | Fannie Mae credit facilities | Fair Value, Measurements, Recurring | Fair Value
Debt instruments - fair value
Fair value		$ 29,034