REAL ESTATE - Schedule of Real Estate Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 2,519,669	$ 2,497,656
Accumulated Depreciation and Amortization	(481,802)	$ (444,299)
Real Estate Investment Property, Net, Total	2,037,867
Domain Gateway | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	63,279
Accumulated Depreciation and Amortization	(11,596)
Real Estate Investment Property, Net, Total	51,683
Town Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	129,070
Accumulated Depreciation and Amortization	(34,895)
Real Estate Investment Property, Net, Total	94,175
McEwen Building | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	37,658
Accumulated Depreciation and Amortization	(9,635)
Real Estate Investment Property, Net, Total	28,023
Gateway Tech Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	28,674
Accumulated Depreciation and Amortization	(7,038)
Real Estate Investment Property, Net, Total	21,636
RBC Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	154,013
Accumulated Depreciation and Amortization	(47,843)
Real Estate Investment Property, Net, Total	106,170
Preston Commons | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	127,822
Accumulated Depreciation and Amortization	(25,266)
Real Estate Investment Property, Net, Total	102,556
Sterling Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	82,951
Accumulated Depreciation and Amortization	(18,730)
Real Estate Investment Property, Net, Total	64,221
201 Spear Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	149,327
Accumulated Depreciation and Amortization	(22,969)
Real Estate Investment Property, Net, Total	126,358
Accenture Tower | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	454,560
Accumulated Depreciation and Amortization	(93,024)
Real Estate Investment Property, Net, Total	361,536
Anchor Centre | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	97,786
Accumulated Depreciation and Amortization	(21,235)
Real Estate Investment Property, Net, Total	76,551
Ten Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	126,863
Accumulated Depreciation and Amortization	(23,717)
Real Estate Investment Property, Net, Total	103,146
Towers at Emeryville | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	209,176
Accumulated Depreciation and Amortization	(37,325)
Real Estate Investment Property, Net, Total	171,851
3003 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	151,372
Accumulated Depreciation and Amortization	(28,021)
Real Estate Investment Property, Net, Total	123,351
Park Place Village | Office/Retail
Real Estate Properties [Line Items]
Total Real Estate at Cost	76,774
Accumulated Depreciation and Amortization	(909)
Real Estate Investment Property, Net, Total	75,865
201 17th Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	103,893
Accumulated Depreciation and Amortization	(21,264)
Real Estate Investment Property, Net, Total	82,629
515 Congress | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	125,885
Accumulated Depreciation and Amortization	(19,299)
Real Estate Investment Property, Net, Total	106,586
The Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	181,714
Accumulated Depreciation and Amortization	(25,736)
Real Estate Investment Property, Net, Total	155,978
3001 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	60,848
Accumulated Depreciation and Amortization	(7,987)
Real Estate Investment Property, Net, Total	52,861
Carillon | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	158,004
Accumulated Depreciation and Amortization	(25,313)
Real Estate Investment Property, Net, Total	$ 132,691