REAL ESTATE (Schedule of Real Estate Investments) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Aug. 26, 2016
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 3,392,797	$ 3,369,925
Accumulated Depreciation and Amortization	(463,090)	(435,808)
Total real estate, net	$ 2,929,707	$ 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	$ 47,482
Accumulated Depreciation and Amortization	(14,074)
Total real estate, net	$ 33,408
Town Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Mar. 27, 2012
Total Real Estate at Cost	$ 115,755
Accumulated Depreciation and Amortization	(25,893)
Total real estate, net	$ 89,862
McEwen Building | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Apr. 30, 2012
Total Real Estate at Cost	$ 37,156
Accumulated Depreciation and Amortization	(7,509)
Total real estate, net	$ 29,647
Gateway Tech Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 09, 2012
Total Real Estate at Cost	$ 24,598
Accumulated Depreciation and Amortization	(5,962)
Total real estate, net	$ 18,636
Tower on Lake Carolyn | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 21, 2012
Total Real Estate at Cost	$ 51,508
Accumulated Depreciation and Amortization	(11,053)
Total real estate, net	$ 40,455
RBC Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jan. 31, 2013
Total Real Estate at Cost	$ 153,814
Accumulated Depreciation and Amortization	(33,155)
Total real estate, net	$ 120,659
One Washingtonian Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 91,463
Accumulated Depreciation and Amortization	(16,274)
Total real estate, net	$ 75,189
Preston Commons | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 118,047
Accumulated Depreciation and Amortization	(20,714)
Total real estate, net	$ 97,333
Sterling Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 19, 2013
Total Real Estate at Cost	$ 79,761
Accumulated Depreciation and Amortization	(12,426)
Total real estate, net	$ 67,335
201 Spear Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 03, 2013
Total Real Estate at Cost	$ 143,531
Accumulated Depreciation and Amortization	(13,265)
Total real estate, net	$ 130,266
500 West Madison | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 16, 2013
Total Real Estate at Cost	$ 438,217
Accumulated Depreciation and Amortization	(65,896)
Total real estate, net	$ 372,321
222 Main | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Feb. 27, 2014
Total Real Estate at Cost	$ 161,387
Accumulated Depreciation and Amortization	(26,283)
Total real estate, net	$ 135,104
Anchor Centre | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 22, 2014
Total Real Estate at Cost	$ 95,149
Accumulated Depreciation and Amortization	(14,402)
Total real estate, net	$ 80,747
171 17th Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 25, 2014
Total Real Estate at Cost	$ 133,415
Accumulated Depreciation and Amortization	(23,055)
Total real estate, net	$ 110,360
Reston Square | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 03, 2014
Total Real Estate at Cost	$ 46,816
Accumulated Depreciation and Amortization	(7,282)
Total real estate, net	$ 39,534
Ten Almaden | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 05, 2014
Total Real Estate at Cost	$ 122,256
Accumulated Depreciation and Amortization	(13,976)
Total real estate, net	$ 108,280
Towers at Emeryville | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 23, 2014
Total Real Estate at Cost	$ 271,090
Accumulated Depreciation and Amortization	(29,530)
Total real estate, net	$ 241,560
101 South Hanley | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 24, 2014
Total Real Estate at Cost	$ 71,720
Accumulated Depreciation and Amortization	(9,466)
Total real estate, net	$ 62,254
3003 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 30, 2014
Total Real Estate at Cost	$ 151,134
Accumulated Depreciation and Amortization	(16,416)
Total real estate, net	$ 134,718
Village Center Station | Office Properties
Real Estate Properties [Line Items]
Date Acquired	May 20, 2015
Total Real Estate at Cost	$ 78,183
Accumulated Depreciation and Amortization	(10,599)
Total real estate, net	$ 67,584
Park Place Village | Office/Retail
Real Estate Properties [Line Items]
Date Acquired	Jun. 18, 2015
Total Real Estate at Cost	$ 128,678
Accumulated Depreciation and Amortization	(14,819)
Total real estate, net	$ 113,859
201 17th Street | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jun. 23, 2015
Total Real Estate at Cost	$ 102,620
Accumulated Depreciation and Amortization	(11,430)
Total real estate, net	$ 91,190
Promenade I & II at Eilan | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jul. 14, 2015
Total Real Estate at Cost	$ 62,643
Accumulated Depreciation and Amortization	(7,497)
Total real estate, net	$ 55,146
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	(9,260)
Total real estate, net	$ 81,092
515 Congress | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 31, 2015
Total Real Estate at Cost	$ 118,766
Accumulated Depreciation and Amortization	(12,265)
Total real estate, net	$ 106,501
The Almaden | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 23, 2015
Total Real Estate at Cost	$ 167,501
Accumulated Depreciation and Amortization	(13,696)
Total real estate, net	$ 153,805
3001 Washington Boulevard | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Nov. 06, 2015
Total Real Estate at Cost	$ 57,135
Accumulated Depreciation and Amortization	(3,532)
Total real estate, net	$ 53,603
Carillon | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Jan. 15, 2016
Total Real Estate at Cost	$ 152,662
Accumulated Depreciation and Amortization	(13,361)
Total real estate, net	$ 139,301
Hardware Village | Development/Apartment
Real Estate Properties [Line Items]
Date Acquired	Aug. 26, 2016
Total Real Estate at Cost	$ 79,958
Accumulated Depreciation and Amortization	0
Total real estate, net	$ 79,958