ACQUISITIONS AND DISPOSITIONS - 2017 Disposals (Details) $ in Thousands												1 Months Ended	3 Months Ended				12 Months Ended
	Dec. 29, 2017 USD ($) ft²	Dec. 15, 2017 USD ($) apartment	Oct. 17, 2017 USD ($) ft²	Sep. 26, 2017 USD ($) apartment	Sep. 21, 2017 USD ($) ft²	Aug. 31, 2017 USD ($) ft²	Jun. 23, 2017 USD ($) apartment	Jun. 20, 2017 USD ($) ft²	Jun. 08, 2017 USD ($) ft²	May 30, 2017 USD ($) apartment	Mar. 28, 2017 USD ($) ft²	Mar. 31, 2019	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership sold																	100.00%
Sales Price																	$ 941,032	$ 0	$ 1,018,476
Gain on Sale													$ 0	$ 302	$ 55,221	$ 377,581	433,104	0	408,098
Impairment of real estate													$ 0	$ 0	$ 2,800	$ 66,200	69,000	$ 0	$ 13,100
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership sold												100.00%							100.00%
Sales Price																	990,996		$ 1,092,737
Transaction Costs																	18,350		13,420
Gain on Sale																	$ 433,104		$ 408,098
211 Main Street, San Francisco, CA (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²											417,266
Sales Price											$ 292,882
Transaction Costs											1,435
Gain on Sale											$ 189,242
3636 McKinney Avenue, Dallas, TX (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of units (in apartments) | apartment										103
Sales Price										$ 20,000
Transaction Costs										177
Gain on Sale										$ 6,631
3839 McKinney Avenue, Dallas, TX (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of units (in apartments) | apartment										75
Sales Price										$ 14,100
Transaction Costs										180
Gain on Sale										$ 4,982
200 S College Street, Charlotte, NC | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²									567,865
Sales Price									$ 148,500
Transaction Costs									833
Gain on Sale									$ 45,906
980 9th and 1010 8th Street, Sacramento, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²								485,926
Sales Price								$ 120,500
Transaction Costs								1,119
Gain on Sale								$ 34,559
4649 Cole Avenue, Dallas, TX (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of units (in apartments) | apartment							334
Sales Price							$ 64,000
Transaction Costs							499
Gain on Sale							$ 28,648
800 N Capitol Street, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²						311,593
Percentage of ownership sold																			100.00%
Sales Price						$ 119,750
Transaction Costs						2,388
Gain on Sale						$ 34,456
7083 Hollywood Boulevard | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²					82,193
Sales Price					$ 42,300
Transaction Costs					584
Gain on Sale					$ 23,810
47 E 34th Street, New York, NY | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of units (in apartments) | apartment				110
Sales Price				$ 80,000
Transaction Costs				3,157
Gain on Sale				$ 16,556
370 L'Enfant Promenade, Washington, D.C. (4) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²			409,897
Sales Price			$ 126,680
Transaction Costs			2,451
Gain on Sale			$ 2,994
4200 Scotland Street, Houston, TX | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of units (in apartments) | apartment		308
Sales Price		$ 64,025
Transaction Costs		597
Gain on Sale		$ 20,314
1130 Howard Street, San Francisco, CA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of ownership acquired	100.00%
Square Feet | ft²	21,194
Purchase price	$ 17,717
Closing costs and assumption of liabilities	$ 1,915