ACQUISITIONS AND DISPOSITIONS - 2019 Disposals (Details) $ in Thousands					1 Months Ended	3 Months Ended				12 Months Ended
	Jul. 30, 2019 USD ($) ft²	May 16, 2019 USD ($) ft²	Mar. 14, 2019 USD ($) ft²	Mar. 01, 2019 USD ($) ft²	Mar. 31, 2019	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) acquistion	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of acquisitions | acquistion										0
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 (Note 2)						$ 0	$ 0	$ 2,800	$ 66,200	69,000	$ 0	$ 13,100
Extinguishment of noncontrolling interests as a result of the sale										113
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
March Oakland Properties, 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]
Square Feet | ft²				975,596
Sales Price				$ 512,016
Transaction Costs				8,971
Gain on Sale				$ 289,779
830 1st 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				247,337
Sales Price				$ 116,550
Transaction Costs				2,438
Gain on Sale				$ 45,710
260 Townsend 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]
Square Feet | ft²			66,682
Sales Price			$ 66,000
Transaction Costs			2,539
Gain on Sale			$ 42,092
1333 Broadway, 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]
Square Feet | ft²		254,523
Sales Price		$ 115,430
Transaction Costs		658
Gain on Sale		$ 55,221
Union Square Properties,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	630,650
Sales Price	$ 181,000
Transaction Costs	3,744
Gain on Sale	$ 302