ACQUISITIONS AND DISPOSITIONS - Disposals (Details) $ in Thousands														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	Nov. 19, 2015 USD ($) ft²	Aug. 31, 2017 USD ($) ft²	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price																							$ 1,012,115	$ 94,568	$ 7,786
Gain on sale of real estate															$ 23,005	$ 74,715	$ 116,283	$ 187,734	$ 0	$ 14,927	$ 0	$ 24,739	401,737	39,666	3,092
Impairment of real estate																							$ 13,100	$ 0	$ 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 | ft²										417,266
Sales Price										$ 292,882
Transaction Costs										2,943
Gain on sale of real estate										187,734
Prepayment penalties										$ 1,508
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 Real Estate Property | apartment									103
Sales Price									$ 20,000
Transaction Costs									1,320
Gain on sale of real estate									5,488
Prepayment penalties									$ 1,143
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 Real Estate Property | apartment									75
Sales Price									$ 14,100
Transaction Costs									938
Gain on sale of real estate									4,224
Prepayment penalties									$ 758
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 | ft²								567,865
Sales Price								$ 148,500
Transaction Costs								833
Gain on sale of real estate								$ 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]
Rentable space | ft²							485,926
Sales Price							$ 120,500
Transaction Costs							1,119
Gain on sale of real estate							$ 34,559
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 Real Estate Property | apartment						334
Sales Price						$ 64,000
Transaction Costs						3,311
Gain on sale of real estate						25,836
Prepayment penalties						$ 2,812
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 | ft²					311,593									311,593
Sales Price					$ 119,750
Transaction Costs					2,388
Gain on sale of real estate					$ 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]
Rentable space | ft²				82,193
Sales Price				$ 42,300
Transaction Costs				584
Gain on sale of real estate				$ 23,670
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 Real Estate Property | apartment			110
Sales Price			$ 80,000
Transaction Costs			3,157
Gain on sale of real estate			$ 16,556
370 L'Enfant Promenade, Washington, D.C. (4)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of real estate														$ 13,100
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 | ft²		409,897
Sales Price		$ 126,680
Transaction Costs		2,451
Gain on sale of real estate		$ 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 Real Estate Property | apartment	308
Sales Price	$ 64,025
Transaction Costs	597
Gain on sale of real estate	$ 20,314
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
Transaction Costs												1,026
Gain on sale of real estate												$ 24,739
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
Transaction Costs											706
Gain on sale of real estate											$ 14,927
500 West Santa Ana Boulevard, Santa Ana, 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]
Percentage of ownership sold													100.00%
Rentable space | ft²													37,116
Sales Price													$ 8,050
Transaction Costs													264
Gain on sale of real estate													$ 3,092