REAL ESTATE - Schedule of Real Estate Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 2,495,736	$ 2,497,656
Accumulated Depreciation and Amortization	(460,609)	$ (444,299)
Real Estate Investment Property, Net, Total	2,035,127
Domain Gateway | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	53,906
Accumulated Depreciation and Amortization	(11,298)
Real Estate Investment Property, Net, Total	42,608
Town Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	128,209
Accumulated Depreciation and Amortization	(33,506)
Real Estate Investment Property, Net, Total	94,703
McEwen Building | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	37,505
Accumulated Depreciation and Amortization	(9,305)
Real Estate Investment Property, Net, Total	28,200
Gateway Tech Center | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	28,892
Accumulated Depreciation and Amortization	(8,096)
Real Estate Investment Property, Net, Total	20,796
RBC Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	153,431
Accumulated Depreciation and Amortization	(46,061)
Real Estate Investment Property, Net, Total	107,370
Preston Commons | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	122,153
Accumulated Depreciation and Amortization	(24,874)
Real Estate Investment Property, Net, Total	97,279
Sterling Plaza | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	82,564
Accumulated Depreciation and Amortization	(17,885)
Real Estate Investment Property, Net, Total	64,679
201 Spear Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	149,225
Accumulated Depreciation and Amortization	(21,579)
Real Estate Investment Property, Net, Total	127,646
Accenture Tower | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	450,871
Accumulated Depreciation and Amortization	(88,644)
Real Estate Investment Property, Net, Total	362,227
Anchor Centre | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	97,493
Accumulated Depreciation and Amortization	(20,149)
Real Estate Investment Property, Net, Total	77,344
Ten Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	126,278
Accumulated Depreciation and Amortization	(22,360)
Real Estate Investment Property, Net, Total	103,918
Towers at Emeryville | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	209,270
Accumulated Depreciation and Amortization	(35,461)
Real Estate Investment Property, Net, Total	173,809
3003 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	151,371
Accumulated Depreciation and Amortization	(26,757)
Real Estate Investment Property, Net, Total	124,614
Park Place Village | Office/Retail
Real Estate Properties [Line Items]
Total Real Estate at Cost	76,679
Accumulated Depreciation and Amortization	0
Real Estate Investment Property, Net, Total	76,679
201 17th Street | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	103,893
Accumulated Depreciation and Amortization	(20,122)
Real Estate Investment Property, Net, Total	83,771
515 Congress | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	125,342
Accumulated Depreciation and Amortization	(18,442)
Real Estate Investment Property, Net, Total	106,900
The Almaden | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	180,856
Accumulated Depreciation and Amortization	(24,594)
Real Estate Investment Property, Net, Total	156,262
3001 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	60,848
Accumulated Depreciation and Amortization	(7,508)
Real Estate Investment Property, Net, Total	53,340
Carillon | Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	156,950
Accumulated Depreciation and Amortization	(23,968)
Real Estate Investment Property, Net, Total	$ 132,982