ACQUISITIONS AND DISPOSITIONS - Disposals (Details)													1 Months Ended	3 Months Ended				12 Months Ended
	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	Jul. 19, 2016 USD ($) room	Feb. 02, 2016 USD ($) room	Mar. 31, 2019 USD ($) property	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property																		0
Sales Price																		$ 0	$ 1,012,115,000	$ 94,568,000
Gain on Sale														$ 23,005,000	$ 74,715,000	$ 116,283,000	$ 187,734,000	0	401,737,000	39,666,000
Impairment of real estate																		$ 0	$ 13,100,000	$ 0
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%
211 Main Street, San Francisco, 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]
Rentable space (in sq ft) | ft²										417,266
Sales Price										$ 292,882,000
Transaction Costs										2,943,000
Gain on Sale										187,734,000
Prepayment penalties										$ 1,508,000
3636 McKinney Avenue, Dallas, 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									103
Sales Price									$ 20,000,000
Transaction Costs									1,320,000
Gain on Sale									5,488,000
Prepayment penalties									$ 1,143,000
3839 McKinney Avenue, Dallas, 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									75
Sales Price									$ 14,100,000
Transaction Costs									938,000
Gain on Sale									4,224,000
Prepayment penalties									$ 758,000
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]
Rentable space (in sq ft) | ft²								567,865
Sales Price								$ 148,500,000
Transaction Costs								833,000
Gain on Sale								$ 45,906,000
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]
Rentable space (in sq ft) | ft²							485,926
Sales Price							$ 120,500,000
Transaction Costs							1,119,000
Gain on Sale							$ 34,559,000
4649 Cole Avenue, Dallas, 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						334
Sales Price						$ 64,000,000
Transaction Costs						3,311,000
Gain on Sale						25,836,000
Prepayment penalties						$ 2,812,000
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]
Percentage of ownership sold																			100.00%
Rentable space (in sq ft) | ft²					311,593
Sales Price					$ 119,750,000
Transaction Costs					2,388,000
Gain on Sale					$ 34,456,000
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]
Rentable space (in sq ft) | ft²				82,193
Sales Price				$ 42,300,000
Transaction Costs				584,000
Gain on Sale				$ 23,670,000
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			$ 80,000,000
Transaction Costs			3,157,000
Gain on Sale			$ 16,556,000
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]
Rentable space (in sq ft) | ft²		409,897
Sales Price		$ 126,680,000
Transaction Costs		2,451,000
Gain on Sale		$ 2,994,000
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	$ 64,025,000
Transaction Costs	597,000
Gain on Sale	$ 20,314,000
Courtyard Oakland, Oakland, 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]
Number of rooms | room												162
Sales Price												$ 43,800,000
Transaction Costs												1,026,000
Gain on Sale												$ 24,739,000
LAX Holiday Inn, Los Angeles, 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]
Number of rooms | room											405
Sales Price											$ 52,500,000
Transaction Costs											706,000
Gain on Sale											$ 14,927,000
Subsequent event | 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%
Subsequent event | Properties Sold March 2019 [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of properties													$ 353,000,000
Subsequent event | Washington, D.C | Office properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property													1
Subsequent event | Oakland, California | Office properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property													3
Subsequent event | Oakland, California | Parking garages
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property													1
Subsequent event | San Francisco, California | Office properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property													1