REAL ESTATE (Schedule of Real Estate Investments) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)	Jul. 31, 2018 unit	Dec. 31, 2017 USD ($)	Aug. 26, 2016
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 3,424,773		$ 3,369,925
Accumulated Depreciation and Amortization	(508,654)		(435,808)
Total real estate held for investment, net	$ 2,916,119		$ 2,934,117
Joint Venture
Real Estate Properties [Line Items]
Equity interest in joint venture				99.24%
Domain Gateway | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 29, 2011
Total Real Estate at Cost	$ 49,177
Accumulated Depreciation and Amortization	(15,150)
Total real estate held for investment, net	$ 34,027
Town Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Mar. 27, 2012
Total Real Estate at Cost	$ 116,280
Accumulated Depreciation and Amortization	(27,923)
Total real estate held for investment, net	$ 88,357
McEwen Building | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Apr. 30, 2012
Total Real Estate at Cost	$ 37,130
Accumulated Depreciation and Amortization	(8,181)
Total real estate held for investment, net	$ 28,949
Gateway Tech Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 09, 2012
Total Real Estate at Cost	$ 25,422
Accumulated Depreciation and Amortization	(6,300)
Total real estate held for investment, net	$ 19,122
Tower on Lake Carolyn | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 21, 2012
Total Real Estate at Cost	$ 52,829
Accumulated Depreciation and Amortization	(11,403)
Total real estate held for investment, net	$ 41,426
RBC Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jan. 31, 2013
Total Real Estate at Cost	$ 153,425
Accumulated Depreciation and Amortization	(36,417)
Total real estate held for investment, net	$ 117,008
One Washingtonian Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 91,953
Accumulated Depreciation and Amortization	(18,130)
Total real estate held for investment, net	$ 73,823
Preston Commons | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 117,737
Accumulated Depreciation and Amortization	(22,638)
Total real estate held for investment, net	$ 95,099
Sterling Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 79,076
Accumulated Depreciation and Amortization	(12,750)
Total real estate held for investment, net	$ 66,326
201 Spear Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 03, 2013
Total Real Estate at Cost	$ 142,663
Accumulated Depreciation and Amortization	(14,838)
Total real estate held for investment, net	$ 127,825
500 West Madison | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 16, 2013
Total Real Estate at Cost	$ 435,174
Accumulated Depreciation and Amortization	(67,343)
Total real estate held for investment, net	$ 367,831
222 Main | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Feb. 27, 2014
Total Real Estate at Cost	$ 165,437
Accumulated Depreciation and Amortization	(29,473)
Total real estate held for investment, net	$ 135,964
Anchor Centre | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 22, 2014
Total Real Estate at Cost	$ 96,085
Accumulated Depreciation and Amortization	(15,380)
Total real estate held for investment, net	$ 80,705
171 17th Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 25, 2014
Total Real Estate at Cost	$ 133,339
Accumulated Depreciation and Amortization	(26,009)
Total real estate held for investment, net	$ 107,330
Reston Square | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 03, 2014
Total Real Estate at Cost	$ 46,579
Accumulated Depreciation and Amortization	(8,111)
Total real estate held for investment, net	$ 38,468
Ten Almaden | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 05, 2014
Total Real Estate at Cost	$ 124,435
Accumulated Depreciation and Amortization	(16,618)
Total real estate held for investment, net	$ 107,817
Towers at Emeryville | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 23, 2014
Total Real Estate at Cost	$ 273,887
Accumulated Depreciation and Amortization	(30,730)
Total real estate held for investment, net	$ 243,157
101 South Hanley | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 24, 2014
Total Real Estate at Cost	$ 72,320
Accumulated Depreciation and Amortization	(11,040)
Total real estate held for investment, net	$ 61,280
3003 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 30, 2014
Total Real Estate at Cost	$ 151,150
Accumulated Depreciation and Amortization	(19,003)
Total real estate held for investment, net	$ 132,147
Village Center Station | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 20, 2015
Total Real Estate at Cost	$ 75,035
Accumulated Depreciation and Amortization	(9,863)
Total real estate held for investment, net	$ 65,172
Park Place Village | Office/Retail
Real Estate Properties [Line Items]
Date Acquired	Jun. 18, 2015
Total Real Estate at Cost	$ 127,930
Accumulated Depreciation and Amortization	(16,836)
Total real estate held for investment, net	$ 111,094
201 17th Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 23, 2015
Total Real Estate at Cost	$ 102,679
Accumulated Depreciation and Amortization	(13,756)
Total real estate held for investment, net	$ 88,923
Promenade I & II at Eilan | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jul. 14, 2015
Total Real Estate at Cost	$ 62,646
Accumulated Depreciation and Amortization	(8,918)
Total real estate held for investment, net	$ 53,728
CrossPoint at Valley Forge | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 18, 2015
Total Real Estate at Cost	$ 90,352
Accumulated Depreciation and Amortization	(11,051)
Total real estate held for investment, net	$ 79,301
515 Congress | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 31, 2015
Total Real Estate at Cost	$ 120,654
Accumulated Depreciation and Amortization	(13,309)
Total real estate held for investment, net	$ 107,345
The Almaden | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 23, 2015
Total Real Estate at Cost	$ 169,116
Accumulated Depreciation and Amortization	(15,813)
Total real estate held for investment, net	$ 153,303
3001 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Nov. 06, 2015
Total Real Estate at Cost	$ 60,560
Accumulated Depreciation and Amortization	(4,511)
Total real estate held for investment, net	$ 56,049
Carillon | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jan. 15, 2016
Total Real Estate at Cost	$ 153,514
Accumulated Depreciation and Amortization	(16,534)
Total real estate held for investment, net	$ 136,980
Hardware Village | Development/Apartment
Real Estate Properties [Line Items]
Date Acquired	Aug. 26, 2016
Total Real Estate at Cost	$ 98,189
Accumulated Depreciation and Amortization	(626)
Total real estate held for investment, net	$ 97,563
Hardware Village, West | Joint Venture
Real Estate Properties [Line Items]
Number of units in real estate property | unit		265