REAL ESTATE - Schedule of Real Estate Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 2,536,160	$ 2,497,656
Accumulated Depreciation and Amortization	(504,815)	$ (444,299)
Real Estate Investment Property, Net, Total	2,031,345
Domain Gateway | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	67,019
Accumulated Depreciation and Amortization	(11,893)
Real Estate Investment Property, Net, Total	55,126
Town Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	129,410
Accumulated Depreciation and Amortization	(36,285)
Real Estate Investment Property, Net, Total	93,125
McEwen Building | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	37,974
Accumulated Depreciation and Amortization	(9,971)
Real Estate Investment Property, Net, Total	28,003
Gateway Tech Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	29,518
Accumulated Depreciation and Amortization	(7,377)
Real Estate Investment Property, Net, Total	22,141
RBC Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	154,513
Accumulated Depreciation and Amortization	(49,689)
Real Estate Investment Property, Net, Total	104,824
Preston Commons | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	131,510
Accumulated Depreciation and Amortization	(26,431)
Real Estate Investment Property, Net, Total	105,079
Sterling Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	84,693
Accumulated Depreciation and Amortization	(19,586)
Real Estate Investment Property, Net, Total	65,107
201 Spear Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	149,339
Accumulated Depreciation and Amortization	(24,359)
Real Estate Investment Property, Net, Total	124,980
Accenture Tower | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	455,677
Accumulated Depreciation and Amortization	(97,479)
Real Estate Investment Property, Net, Total	358,198
Anchor Centre | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	97,484
Accumulated Depreciation and Amortization	(22,011)
Real Estate Investment Property, Net, Total	75,473
Ten Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	127,227
Accumulated Depreciation and Amortization	(25,099)
Real Estate Investment Property, Net, Total	102,128
Towers at Emeryville | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	209,141
Accumulated Depreciation and Amortization	(39,263)
Real Estate Investment Property, Net, Total	169,878
3003 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	151,372
Accumulated Depreciation and Amortization	(29,284)
Real Estate Investment Property, Net, Total	122,088
Park Place Village | Office/Retail
Real Estate Properties [Line Items]
Total Real Estate at Cost	76,875
Accumulated Depreciation and Amortization	(1,766)
Real Estate Investment Property, Net, Total	75,109
201 17th Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	104,003
Accumulated Depreciation and Amortization	(22,413)
Real Estate Investment Property, Net, Total	81,590
515 Congress | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	125,254
Accumulated Depreciation and Amortization	(19,650)
Real Estate Investment Property, Net, Total	105,604
The Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	185,080
Accumulated Depreciation and Amortization	(27,534)
Real Estate Investment Property, Net, Total	157,546
3001 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	60,848
Accumulated Depreciation and Amortization	(8,466)
Real Estate Investment Property, Net, Total	52,382
Carillon | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	159,223
Accumulated Depreciation and Amortization	(26,259)
Real Estate Investment Property, Net, Total	$ 132,964