Real Estate and Accumulated Depreciation Schedule of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	[1]	$ 2,979,281
Original Land		4,762,002
Original Building		11,634,050
Costs Capitalized Subsequent to Acquisition		4,662,662
Land and improvements		5,080,679
Buildings and Improvements		14,503,772
Land Held for Development	[2]	204,925
Development and Construction in Progress		1,269,338
Total		21,058,714
Accumulated Depreciation		4,566,570
Furniture, fixtures and equipment		37,928	$ 32,687
Furniture, fixtures and equipment accumulated depreciation		23,064
Real Estate Aggregate Cost For Tax Purposes		17,300,000
Real Estate Aggregate Accumulated Depreciation For Tax Purposes		$ 3,600,000
Real Estate and Accumulated Depreciation, Life Used for Depreciation		40 years
Unamortized deferred finance costs		$ (34,680)
767 Fifth Avenue (the General Motors Building)
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		767 Fifth Avenue (the General Motors Building)
Type		Office
Location		New York, NY
Encumbrances		$ 2,267,041
Original Land		1,796,252
Original Building		1,532,654
Costs Capitalized Subsequent to Acquisition		135,559
Land and improvements		1,796,252
Buildings and Improvements		1,668,213
Land Held for Development		0
Development and Construction in Progress		0
Total		3,464,465
Accumulated Depreciation		$ 222,981
Year(s) Built / Renovated		1968
Year(s) Acquired		2013
Depreciable Lives (Years)	[3]	-1
Prudential Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Prudential Center
Type		Office
Location		Boston, MA
Encumbrances		$ 0
Original Land		92,077
Original Building		948,357
Costs Capitalized Subsequent to Acquisition		501,709
Land and improvements		115,638
Buildings and Improvements		1,426,505
Land Held for Development		0
Development and Construction in Progress		0
Total		1,542,143
Accumulated Depreciation		$ 511,605
Year(s) Built / Renovated		1965/1993/2002/2016-2017
Year(s) Acquired		1998/1999/2000
Depreciable Lives (Years)	[3]	-1
Embarcadero Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Embarcadero Center
Type		Office
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		179,697
Original Building		847,410
Costs Capitalized Subsequent to Acquisition		366,780
Land and improvements		195,987
Buildings and Improvements		1,197,900
Land Held for Development		0
Development and Construction in Progress		0
Total		1,393,887
Accumulated Depreciation		$ 596,100
Year(s) Built / Renovated		1970/1989
Year(s) Acquired		1998-1999
Depreciable Lives (Years)	[3]	-1
399 Park Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		399 Park Avenue
Type		Office
Location		New York, NY
Encumbrances		$ 0
Original Land		339,200
Original Building		700,358
Costs Capitalized Subsequent to Acquisition		181,418
Land and improvements		354,107
Buildings and Improvements		866,869
Land Held for Development		0
Development and Construction in Progress		0
Total		1,220,976
Accumulated Depreciation		$ 310,262
Year(s) Built / Renovated		1961
Year(s) Acquired		2002
Depreciable Lives (Years)	[3]	-1
601 Lexington Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		601 Lexington Avenue
Type		Office
Location		New York, NY
Encumbrances		$ 672,142
Original Land		241,600
Original Building		494,782
Costs Capitalized Subsequent to Acquisition		290,590
Land and improvements		289,639
Buildings and Improvements		634,134
Land Held for Development		0
Development and Construction in Progress		103,199
Total		1,026,972
Accumulated Depreciation		$ 254,932
Year(s) Built / Renovated		1977/1997
Year(s) Acquired		2001
Depreciable Lives (Years)	[3]	-1
200 Clarendon Street and Garage
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		200 Clarendon Street and Garage
Type		Office
Location		Boston, MA
Encumbrances		$ 0
Original Land		219,543
Original Building		667,884
Costs Capitalized Subsequent to Acquisition		139,189
Land and improvements		219,616
Buildings and Improvements		799,991
Land Held for Development		7,009
Development and Construction in Progress		0
Total		1,026,616
Accumulated Depreciation		$ 161,471
Year(s) Built / Renovated		1976
Year(s) Acquired		2010
Depreciable Lives (Years)	[3]	-1
250 West 55th Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		250 West 55th Street
Type		Office
Location		New York, NY
Encumbrances		$ 0
Original Land		285,263
Original Building		603,167
Costs Capitalized Subsequent to Acquisition		43,296
Land and improvements		285,263
Buildings and Improvements		646,463
Land Held for Development		0
Development and Construction in Progress		0
Total		931,726
Accumulated Depreciation		$ 72,058
Year(s) Built / Renovated		2014
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
Carnegie Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Carnegie Center
Type		Office
Location		Princeton, NJ
Encumbrances		$ 0
Original Land		107,997
Original Building		389,359
Costs Capitalized Subsequent to Acquisition		157,960
Land and improvements		108,948
Buildings and Improvements		543,514
Land Held for Development		2,854
Development and Construction in Progress		0
Total		655,316
Accumulated Depreciation		$ 228,171
Year(s) Built / Renovated		1983-2016
Year(s) Acquired		1998/1999/2000/2007/2014/2017
Depreciable Lives (Years)	[3]	-1
100 Federal Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		100 Federal Street
Type		Office
Location		Boston, MA
Encumbrances		$ 0
Original Land		131,067
Original Building		435,954
Costs Capitalized Subsequent to Acquisition		79,035
Land and improvements		131,067
Buildings and Improvements		514,989
Land Held for Development		0
Development and Construction in Progress		0
Total		646,056
Accumulated Depreciation		$ 97,630
Year(s) Built / Renovated		1971-1975/2017
Year(s) Acquired		2012
Depreciable Lives (Years)	[3]	-1
Times Square Tower
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Times Square Tower
Type		Office
Location		New York, NY
Encumbrances		$ 0
Original Land		165,413
Original Building		380,438
Costs Capitalized Subsequent to Acquisition		87,050
Land and improvements		169,193
Buildings and Improvements		463,708
Land Held for Development		0
Development and Construction in Progress		0
Total		632,901
Accumulated Depreciation		$ 191,435
Year(s) Built / Renovated		2004
Year(s) Acquired		2000
Depreciable Lives (Years)	[3]	-1
Atlantic Wharf
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Atlantic Wharf
Type		Office
Location		Boston, MA
Encumbrances		$ 0
Original Land		63,988
Original Building		454,537
Costs Capitalized Subsequent to Acquisition		17,473
Land and improvements		63,988
Buildings and Improvements		472,010
Land Held for Development		0
Development and Construction in Progress		0
Total		535,998
Accumulated Depreciation		$ 101,525
Year(s) Built / Renovated		2011
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
Fountain Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Fountain Square
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		56,853
Original Building		306,298
Costs Capitalized Subsequent to Acquisition		17,834
Land and improvements		56,853
Buildings and Improvements		320,474
Land Held for Development		3,658
Development and Construction in Progress		0
Total		380,985
Accumulated Depreciation		$ 60,098
Year(s) Built / Renovated		1986-1990
Year(s) Acquired		2012
Depreciable Lives (Years)	[3]	-1
510 Madison Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		510 Madison Avenue
Type		Office
Location		New York, NY
Encumbrances		$ 0
Original Land		103,000
Original Building		253,665
Costs Capitalized Subsequent to Acquisition		23,875
Land and improvements		103,000
Buildings and Improvements		277,540
Land Held for Development		0
Development and Construction in Progress		0
Total		380,540
Accumulated Depreciation		$ 54,056
Year(s) Built / Renovated		2012
Year(s) Acquired		2010
Depreciable Lives (Years)	[3]	-1
599 Lexington Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		599 Lexington Avenue
Type		Office
Location		New York, NY
Encumbrances		$ 0
Original Land		81,040
Original Building		100,507
Costs Capitalized Subsequent to Acquisition		165,350
Land and improvements		87,852
Buildings and Improvements		259,045
Land Held for Development		0
Development and Construction in Progress		0
Total		346,897
Accumulated Depreciation		$ 165,739
Year(s) Built / Renovated		1986
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
680 Folsom Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		680 Folsom Street
Type		Office
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		72,545
Original Building		219,766
Costs Capitalized Subsequent to Acquisition		7,545
Land and improvements		72,545
Buildings and Improvements		227,311
Land Held for Development		0
Development and Construction in Progress		0
Total		299,856
Accumulated Depreciation		$ 30,577
Year(s) Built / Renovated		2014
Year(s) Acquired		2012
Depreciable Lives (Years)	[3]	-1
South of Market and Democracy Tower
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		South of Market and Democracy Tower
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		13,603
Original Building		237,479
Costs Capitalized Subsequent to Acquisition		14,991
Land and improvements		13,687
Buildings and Improvements		252,386
Land Held for Development		0
Development and Construction in Progress		0
Total		266,073
Accumulated Depreciation		$ 87,711
Year(s) Built / Renovated		2008-2009
Year(s) Acquired		2003
Depreciable Lives (Years)	[3]	-1
601 Massachusetts Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		601 Massachusetts Avenue
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		95,310
Original Building		165,173
Costs Capitalized Subsequent to Acquisition		2,353
Land and improvements		95,322
Buildings and Improvements		167,514
Land Held for Development		0
Development and Construction in Progress		0
Total		262,836
Accumulated Depreciation		$ 12,177
Year(s) Built / Renovated		2016
Year(s) Acquired		2008
Depreciable Lives (Years)	[3]	-1
Bay Colony Corporate Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Bay Colony Corporate Center
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		18,789
Original Building		148,451
Costs Capitalized Subsequent to Acquisition		73,418
Land and improvements		18,789
Buildings and Improvements		221,869
Land Held for Development		0
Development and Construction in Progress		0
Total		240,658
Accumulated Depreciation		$ 57,320
Year(s) Built / Renovated		1985-1989
Year(s) Acquired		2011
Depreciable Lives (Years)	[3]	-1
Gateway Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Gateway Center
Type		Office
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		28,255
Original Building		139,245
Costs Capitalized Subsequent to Acquisition		59,019
Land and improvements		30,627
Buildings and Improvements		195,892
Land Held for Development		0
Development and Construction in Progress		0
Total		226,519
Accumulated Depreciation		$ 102,634
Year(s) Built / Renovated		1984/1986/2002
Year(s) Acquired		1999
Depreciable Lives (Years)	[3]	-1
535 Mission Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		535 Mission Street
Type		Office
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		40,933
Original Building		148,378
Costs Capitalized Subsequent to Acquisition		3,259
Land and improvements		40,933
Buildings and Improvements		151,637
Land Held for Development		0
Development and Construction in Progress		0
Total		192,570
Accumulated Depreciation		$ 14,637
Year(s) Built / Renovated		2015
Year(s) Acquired		2013
Depreciable Lives (Years)	[3]	-1
2200 Pennsylvania Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		2200 Pennsylvania Avenue
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		0
Original Building		183,541
Costs Capitalized Subsequent to Acquisition		4,907
Land and improvements		0
Buildings and Improvements		188,448
Land Held for Development		0
Development and Construction in Progress		0
Total		188,448
Accumulated Depreciation		$ 46,780
Year(s) Built / Renovated		2011
Year(s) Acquired		2008
Depreciable Lives (Years)	[3]	-1
Mountain View Research Park
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Mountain View Research Park
Type		Office
Location		Mountain View, CA
Encumbrances		$ 0
Original Land		95,066
Original Building		68,373
Costs Capitalized Subsequent to Acquisition		8,136
Land and improvements		95,066
Buildings and Improvements		76,509
Land Held for Development		0
Development and Construction in Progress		0
Total		171,575
Accumulated Depreciation		$ 15,146
Year(s) Built / Renovated		1977-1981/2007-2013
Year(s) Acquired		2013
Depreciable Lives (Years)	[3]	-1
Reservoir Place
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Reservoir Place
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		18,605
Original Building		104,124
Costs Capitalized Subsequent to Acquisition		43,255
Land and improvements		20,108
Buildings and Improvements		145,876
Land Held for Development		0
Development and Construction in Progress		0
Total		165,984
Accumulated Depreciation		$ 66,230
Year(s) Built / Renovated		1955/1987/2017
Year(s) Acquired		1997/1998
Depreciable Lives (Years)	[3]	-1
1330 Connecticut Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		1330 Connecticut Avenue
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		25,982
Original Building		82,311
Costs Capitalized Subsequent to Acquisition		32,839
Land and improvements		27,135
Buildings and Improvements		113,997
Land Held for Development		0
Development and Construction in Progress		0
Total		141,132
Accumulated Depreciation		$ 22,530
Year(s) Built / Renovated		1984
Year(s) Acquired		2004
Depreciable Lives (Years)	[3]	-1
1333 New Hampshire Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		1333 New Hampshire Avenue
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		34,032
Original Building		85,660
Costs Capitalized Subsequent to Acquisition		11,473
Land and improvements		35,382
Buildings and Improvements		95,783
Land Held for Development		0
Development and Construction in Progress		0
Total		131,165
Accumulated Depreciation		$ 40,499
Year(s) Built / Renovated		1996
Year(s) Acquired		2003
Depreciable Lives (Years)	[3]	-1
Kingstowne Towne Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Kingstowne Towne Center
Type		Office
Location		Alexandria, VA
Encumbrances		$ 0
Original Land		18,021
Original Building		109,038
Costs Capitalized Subsequent to Acquisition		1,371
Land and improvements		18,062
Buildings and Improvements		110,368
Land Held for Development		0
Development and Construction in Progress		0
Total		128,430
Accumulated Depreciation		$ 39,048
Year(s) Built / Renovated		2003-2006
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
One Freedom Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		One Freedom Square
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		9,929
Original Building		84,504
Costs Capitalized Subsequent to Acquisition		32,618
Land and improvements		11,293
Buildings and Improvements		115,758
Land Held for Development		0
Development and Construction in Progress		0
Total		127,051
Accumulated Depreciation		$ 50,313
Year(s) Built / Renovated		2000
Year(s) Acquired		2003
Depreciable Lives (Years)	[3]	-1
Capital Gallery
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Capital Gallery
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		4,725
Original Building		29,565
Costs Capitalized Subsequent to Acquisition		89,514
Land and improvements		8,662
Buildings and Improvements		115,142
Land Held for Development		0
Development and Construction in Progress		0
Total		123,804
Accumulated Depreciation		$ 66,009
Year(s) Built / Renovated		1981/2006
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
Weston Corporate Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Weston Corporate Center
Type		Office
Location		Weston, MA
Encumbrances		$ 0
Original Land		25,753
Original Building		92,312
Costs Capitalized Subsequent to Acquisition		(123)
Land and improvements		25,854
Buildings and Improvements		92,088
Land Held for Development		0
Development and Construction in Progress		0
Total		117,942
Accumulated Depreciation		$ 23,125
Year(s) Built / Renovated		2010
Year(s) Acquired		2001
Depreciable Lives (Years)	[3]	-1
Two Freedom Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Two Freedom Square
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		13,930
Original Building		77,739
Costs Capitalized Subsequent to Acquisition		23,379
Land and improvements		15,420
Buildings and Improvements		99,628
Land Held for Development		0
Development and Construction in Progress		0
Total		115,048
Accumulated Depreciation		$ 47,936
Year(s) Built / Renovated		2001
Year(s) Acquired		2003
Depreciable Lives (Years)	[3]	-1
One and Two Reston Overlook
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		One and Two Reston Overlook
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		16,456
Original Building		66,192
Costs Capitalized Subsequent to Acquisition		25,111
Land and improvements		16,179
Buildings and Improvements		91,580
Land Held for Development		0
Development and Construction in Progress		0
Total		107,759
Accumulated Depreciation		$ 44,626
Year(s) Built / Renovated		1999
Year(s) Acquired		2000
Depreciable Lives (Years)	[3]	-1
Discovery Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Discovery Square
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		11,198
Original Building		71,782
Costs Capitalized Subsequent to Acquisition		24,414
Land and improvements		12,533
Buildings and Improvements		94,861
Land Held for Development		0
Development and Construction in Progress		0
Total		107,394
Accumulated Depreciation		$ 43,184
Year(s) Built / Renovated		2001
Year(s) Acquired		2003
Depreciable Lives (Years)	[3]	-1
140 Kendrick Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		140 Kendrick Street
Type		Office
Location		Needham, MA
Encumbrances		$ 0
Original Land		18,095
Original Building		66,905
Costs Capitalized Subsequent to Acquisition		17,492
Land and improvements		19,092
Buildings and Improvements		83,400
Land Held for Development		0
Development and Construction in Progress		0
Total		102,492
Accumulated Depreciation		$ 29,013
Year(s) Built / Renovated		2000
Year(s) Acquired		2004
Depreciable Lives (Years)	[3]	-1
355 Main Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		355 Main Street
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		18,863
Original Building		53,346
Costs Capitalized Subsequent to Acquisition		27,450
Land and improvements		21,173
Buildings and Improvements		78,486
Land Held for Development		0
Development and Construction in Progress		0
Total		99,659
Accumulated Depreciation		$ 26,915
Year(s) Built / Renovated		1981/1996/2013
Year(s) Acquired		2006
Depreciable Lives (Years)	[3]	-1
10 CityPoint
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		10 CityPoint
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		1,953
Original Building		85,752
Costs Capitalized Subsequent to Acquisition		2,833
Land and improvements		2,116
Buildings and Improvements		88,422
Land Held for Development		0
Development and Construction in Progress		0
Total		90,538
Accumulated Depreciation		$ 4,673
Year(s) Built / Renovated		2016
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
90 Broadway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		90 Broadway
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		19,104
Original Building		52,078
Costs Capitalized Subsequent to Acquisition		17,180
Land and improvements		20,785
Buildings and Improvements		67,577
Land Held for Development		0
Development and Construction in Progress		0
Total		88,362
Accumulated Depreciation		$ 19,254
Year(s) Built / Renovated		1983/1998/2013
Year(s) Acquired		2006
Depreciable Lives (Years)	[3]	-1
230 CityPoint
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		230 CityPoint
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		13,189
Original Building		49,823
Costs Capitalized Subsequent to Acquisition		25,236
Land and improvements		13,593
Buildings and Improvements		74,655
Land Held for Development		0
Development and Construction in Progress		0
Total		88,248
Accumulated Depreciation		$ 26,562
Year(s) Built / Renovated		1992
Year(s) Acquired		2005
Depreciable Lives (Years)	[3]	-1
Waltham Weston Corporate Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Waltham Weston Corporate Center
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		10,385
Original Building		60,694
Costs Capitalized Subsequent to Acquisition		11,030
Land and improvements		11,097
Buildings and Improvements		71,012
Land Held for Development		0
Development and Construction in Progress		0
Total		82,109
Accumulated Depreciation		$ 30,036
Year(s) Built / Renovated		2003
Year(s) Acquired		1999
Depreciable Lives (Years)	[3]	-1
77 CityPoint
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		77 CityPoint
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		13,847
Original Building		60,383
Costs Capitalized Subsequent to Acquisition		5,703
Land and improvements		13,873
Buildings and Improvements		66,060
Land Held for Development		0
Development and Construction in Progress		0
Total		79,933
Accumulated Depreciation		$ 23,044
Year(s) Built / Renovated		2008
Year(s) Acquired		2001
Depreciable Lives (Years)	[3]	-1
3625-3635 Peterson Way
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		3625-3635 Peterson Way
Type		Office
Location		Santa Clara, CA
Encumbrances		$ 0
Original Land		63,206
Original Building		14,879
Costs Capitalized Subsequent to Acquisition		138
Land and improvements		63,206
Buildings and Improvements		14,879
Land Held for Development		138
Development and Construction in Progress		0
Total		78,223
Accumulated Depreciation		$ 5,120
Year(s) Built / Renovated		1979
Year(s) Acquired		2016
Depreciable Lives (Years)	[3]	-1
North First Business Park
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		North First Business Park
Type		Office
Location		San Jose, CA
Encumbrances		$ 0
Original Land		58,402
Original Building		13,069
Costs Capitalized Subsequent to Acquisition		4,416
Land and improvements		23,377
Buildings and Improvements		16,600
Land Held for Development		35,910
Development and Construction in Progress		0
Total		75,887
Accumulated Depreciation		$ 15,839
Year(s) Built / Renovated		1981
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
300 Binney Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		300 Binney Street
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		18,080
Original Building		51,262
Costs Capitalized Subsequent to Acquisition		140
Land and improvements		18,080
Buildings and Improvements		51,402
Land Held for Development		0
Development and Construction in Progress		0
Total		69,482
Accumulated Depreciation		$ 7,755
Year(s) Built / Renovated		2013
Year(s) Acquired		2009
Depreciable Lives (Years)	[3]	(1)
2440 West El Camino Real
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		2440 West El Camino Real
Type		Office
Location		Mountain View, CA
Encumbrances		$ 0
Original Land		16,741
Original Building		51,285
Costs Capitalized Subsequent to Acquisition		1,287
Land and improvements		16,741
Buildings and Improvements		52,572
Land Held for Development		0
Development and Construction in Progress		0
Total		69,313
Accumulated Depreciation		$ 10,790
Year(s) Built / Renovated		1987/2003
Year(s) Acquired		2011
Depreciable Lives (Years)	[3]	-1
Wisconsin Place
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Wisconsin Place
Type		Office
Location		Chevy Chase, MD
Encumbrances		$ 0
Original Land		0
Original Building		53,349
Costs Capitalized Subsequent to Acquisition		14,907
Land and improvements		0
Buildings and Improvements		68,256
Land Held for Development		0
Development and Construction in Progress		0
Total		68,256
Accumulated Depreciation		$ 19,775
Year(s) Built / Renovated		2009
Year(s) Acquired		2004
Depreciable Lives (Years)	[3]	-1
Reston Corporate Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Reston Corporate Center
Type		Office
Location		Reston, VA
Encumbrances		$ 0
Original Land		9,135
Original Building		50,857
Costs Capitalized Subsequent to Acquisition		6,236
Land and improvements		10,148
Buildings and Improvements		56,080
Land Held for Development		0
Development and Construction in Progress		0
Total		66,228
Accumulated Depreciation		$ 26,769
Year(s) Built / Renovated		1984
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
New Dominion Technology Park, Bldg. Two
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		New Dominion Technology Park, Bldg. Two
Type		Office
Location		Herndon, VA
Encumbrances		$ 0
Original Land		5,584
Original Building		51,868
Costs Capitalized Subsequent to Acquisition		4,157
Land and improvements		6,510
Buildings and Improvements		55,099
Land Held for Development		0
Development and Construction in Progress		0
Total		61,609
Accumulated Depreciation		$ 23,652
Year(s) Built / Renovated		2004
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
200 West Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		200 West Street
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		16,148
Original Building		24,983
Costs Capitalized Subsequent to Acquisition		10,673
Land and improvements		16,813
Buildings and Improvements		34,991
Land Held for Development		0
Development and Construction in Progress		0
Total		51,804
Accumulated Depreciation		$ 21,140
Year(s) Built / Renovated		1999
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
New Dominion Technology Park, Bldg. One
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		New Dominion Technology Park, Bldg. One
Type		Office
Location		Herndon, VA
Encumbrances		$ 32,691
Original Land		3,880
Original Building		43,227
Costs Capitalized Subsequent to Acquisition		3,882
Land and improvements		4,583
Buildings and Improvements		46,406
Land Held for Development		0
Development and Construction in Progress		0
Total		50,989
Accumulated Depreciation		$ 25,940
Year(s) Built / Renovated		2001
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
Sumner Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Sumner Square
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		624
Original Building		28,745
Costs Capitalized Subsequent to Acquisition		19,000
Land and improvements		1,478
Buildings and Improvements		46,891
Land Held for Development		0
Development and Construction in Progress		0
Total		48,369
Accumulated Depreciation		$ 23,570
Year(s) Built / Renovated		1985
Year(s) Acquired		1999
Depreciable Lives (Years)	[3]	-1
255 Main Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		255 Main Street
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		134
Original Building		25,110
Costs Capitalized Subsequent to Acquisition		21,583
Land and improvements		548
Buildings and Improvements		46,279
Land Held for Development		0
Development and Construction in Progress		0
Total		46,827
Accumulated Depreciation		$ 28,250
Year(s) Built / Renovated		1987
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
University Place
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		University Place
Type		Office
Location		Cambridge, MA
Encumbrances		$ 7,407
Original Land		0
Original Building		37,091
Costs Capitalized Subsequent to Acquisition		9,007
Land and improvements		390
Buildings and Improvements		45,708
Land Held for Development		0
Development and Construction in Progress		0
Total		46,098
Accumulated Depreciation		$ 26,295
Year(s) Built / Renovated		1985
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
2600 Tower Oaks Boulevard
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		2600 Tower Oaks Boulevard
Type		Office
Location		Rockville, MD
Encumbrances		$ 0
Original Land		4,243
Original Building		31,125
Costs Capitalized Subsequent to Acquisition		9,867
Land and improvements		4,785
Buildings and Improvements		40,450
Land Held for Development		0
Development and Construction in Progress		0
Total		45,235
Accumulated Depreciation		$ 21,020
Year(s) Built / Renovated		2001
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
Quorum Office Park
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Quorum Office Park
Type		Office
Location		Chelmsford, MA
Encumbrances		$ 0
Original Land		3,750
Original Building		32,454
Costs Capitalized Subsequent to Acquisition		5,813
Land and improvements		5,187
Buildings and Improvements		36,830
Land Held for Development		0
Development and Construction in Progress		0
Total		42,017
Accumulated Depreciation		$ 16,594
Year(s) Built / Renovated		2001
Year(s) Acquired		2000
Depreciable Lives (Years)	[3]	-1
500 E Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		500 E Street
Type		Office
Location		Washington, DC
Encumbrances		$ 0
Original Land		109
Original Building		22,420
Costs Capitalized Subsequent to Acquisition		14,743
Land and improvements		2,379
Buildings and Improvements		34,893
Land Held for Development		0
Development and Construction in Progress		0
Total		37,272
Accumulated Depreciation		$ 23,781
Year(s) Built / Renovated		1987
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
150 Broadway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		150 Broadway
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		850
Original Building		25,042
Costs Capitalized Subsequent to Acquisition		8,540
Land and improvements		1,323
Buildings and Improvements		33,109
Land Held for Development		0
Development and Construction in Progress		0
Total		34,432
Accumulated Depreciation		$ 16,933
Year(s) Built / Renovated		1999
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
325 Main Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		325 Main Street
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		174
Original Building		12,200
Costs Capitalized Subsequent to Acquisition		11,963
Land and improvements		965
Buildings and Improvements		23,372
Land Held for Development		0
Development and Construction in Progress		0
Total		24,337
Accumulated Depreciation		$ 12,852
Year(s) Built / Renovated		1987/2013
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
105 Broadway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		105 Broadway
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		1,299
Original Building		12,943
Costs Capitalized Subsequent to Acquisition		8,214
Land and improvements		2,395
Buildings and Improvements		20,061
Land Held for Development		0
Development and Construction in Progress		0
Total		22,456
Accumulated Depreciation		$ 13,493
Year(s) Built / Renovated		1990
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
Lexington Office Park
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Lexington Office Park
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		998
Original Building		1,426
Costs Capitalized Subsequent to Acquisition		18,219
Land and improvements		1,264
Buildings and Improvements		19,379
Land Held for Development		0
Development and Construction in Progress		0
Total		20,643
Accumulated Depreciation		$ 13,404
Year(s) Built / Renovated		1982
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
201 Spring Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		201 Spring Street
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		2,849
Original Building		15,303
Costs Capitalized Subsequent to Acquisition		694
Land and improvements		3,124
Buildings and Improvements		15,722
Land Held for Development		0
Development and Construction in Progress		0
Total		18,846
Accumulated Depreciation		$ 7,931
Year(s) Built / Renovated		1997
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
The Point
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		The Point
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		6,395
Original Building		10,040
Costs Capitalized Subsequent to Acquisition		409
Land and improvements		6,480
Buildings and Improvements		10,364
Land Held for Development		0
Development and Construction in Progress		0
Total		16,844
Accumulated Depreciation		$ 714
Year(s) Built / Renovated		2015
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
92-100 Hayden Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		92-100 Hayden Avenue
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		594
Original Building		6,748
Costs Capitalized Subsequent to Acquisition		8,181
Land and improvements		802
Buildings and Improvements		14,721
Land Held for Development		0
Development and Construction in Progress		0
Total		15,523
Accumulated Depreciation		$ 12,495
Year(s) Built / Renovated		1985
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
91 Hartwell Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		91 Hartwell Avenue
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		784
Original Building		6,464
Costs Capitalized Subsequent to Acquisition		8,269
Land and improvements		941
Buildings and Improvements		14,576
Land Held for Development		0
Development and Construction in Progress		0
Total		15,517
Accumulated Depreciation		$ 9,849
Year(s) Built / Renovated		1985
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
690 Folsom Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		690 Folsom Street
Type		Office
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		3,219
Original Building		11,038
Costs Capitalized Subsequent to Acquisition		1,157
Land and improvements		3,219
Buildings and Improvements		12,195
Land Held for Development		0
Development and Construction in Progress		0
Total		15,414
Accumulated Depreciation		$ 1,295
Year(s) Built / Renovated		2015
Year(s) Acquired		2012
Depreciable Lives (Years)	[3]	-1
181 Spring Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		181 Spring Street
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		1,066
Original Building		9,520
Costs Capitalized Subsequent to Acquisition		2,318
Land and improvements		1,160
Buildings and Improvements		11,744
Land Held for Development		0
Development and Construction in Progress		0
Total		12,904
Accumulated Depreciation		$ 5,194
Year(s) Built / Renovated		1999
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
195 West Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		195 West Street
Type		Office
Location		Waltham, MA
Encumbrances		$ 0
Original Land		1,611
Original Building		6,652
Costs Capitalized Subsequent to Acquisition		4,340
Land and improvements		1,858
Buildings and Improvements		10,745
Land Held for Development		0
Development and Construction in Progress		0
Total		12,603
Accumulated Depreciation		$ 7,963
Year(s) Built / Renovated		1990
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
33 Hayden Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		33 Hayden Avenue
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		266
Original Building		3,234
Costs Capitalized Subsequent to Acquisition		8,381
Land and improvements		425
Buildings and Improvements		11,456
Land Held for Development		0
Development and Construction in Progress		0
Total		11,881
Accumulated Depreciation		$ 8,211
Year(s) Built / Renovated		1979
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7501 Boston Boulevard, Building Seven
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7501 Boston Boulevard, Building Seven
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		665
Original Building		9,273
Costs Capitalized Subsequent to Acquisition		819
Land and improvements		791
Buildings and Improvements		9,966
Land Held for Development		0
Development and Construction in Progress		0
Total		10,757
Accumulated Depreciation		$ 4,848
Year(s) Built / Renovated		1997
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7435 Boston Boulevard, Building One
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7435 Boston Boulevard, Building One
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		392
Original Building		3,822
Costs Capitalized Subsequent to Acquisition		3,997
Land and improvements		659
Buildings and Improvements		7,552
Land Held for Development		0
Development and Construction in Progress		0
Total		8,211
Accumulated Depreciation		$ 5,510
Year(s) Built / Renovated		1982
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
250 Binney Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		250 Binney Street
Type		Office
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		110
Original Building		4,483
Costs Capitalized Subsequent to Acquisition		3,593
Land and improvements		273
Buildings and Improvements		7,913
Land Held for Development		0
Development and Construction in Progress		0
Total		8,186
Accumulated Depreciation		$ 5,066
Year(s) Built / Renovated		1983
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
8000 Grainger Court, Building Five
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		8000 Grainger Court, Building Five
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		366
Original Building		4,282
Costs Capitalized Subsequent to Acquisition		3,375
Land and improvements		601
Buildings and Improvements		7,422
Land Held for Development		0
Development and Construction in Progress		0
Total		8,023
Accumulated Depreciation		$ 5,644
Year(s) Built / Renovated		1984
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7450 Boston Boulevard, Building Three
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7450 Boston Boulevard, Building Three
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		1,165
Original Building		4,681
Costs Capitalized Subsequent to Acquisition		1,773
Land and improvements		1,430
Buildings and Improvements		6,189
Land Held for Development		0
Development and Construction in Progress		0
Total		7,619
Accumulated Depreciation		$ 3,451
Year(s) Built / Renovated		1987
Year(s) Acquired		1998
Depreciable Lives (Years)	[3]	-1
453 Ravendale Drive
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		453 Ravendale Drive
Type		Office
Location		Mountain View, CA
Encumbrances		$ 0
Original Land		5,477
Original Building		1,090
Costs Capitalized Subsequent to Acquisition		408
Land and improvements		5,477
Buildings and Improvements		1,498
Land Held for Development		0
Development and Construction in Progress		0
Total		6,975
Accumulated Depreciation		$ 472
Year(s) Built / Renovated		1977
Year(s) Acquired		2012
Depreciable Lives (Years)	[3]	-1
17 Hartwell Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		17 Hartwell Avenue
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		26
Original Building		150
Costs Capitalized Subsequent to Acquisition		6,252
Land and improvements		65
Buildings and Improvements		6,363
Land Held for Development		0
Development and Construction in Progress		0
Total		6,428
Accumulated Depreciation		$ 1,098
Year(s) Built / Renovated		1968
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7601 Boston Boulevard, Building Eight
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7601 Boston Boulevard, Building Eight
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		200
Original Building		878
Costs Capitalized Subsequent to Acquisition		5,052
Land and improvements		551
Buildings and Improvements		5,579
Land Held for Development		0
Development and Construction in Progress		0
Total		6,130
Accumulated Depreciation		$ 4,263
Year(s) Built / Renovated		1986
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7300 Boston Boulevard, Building Thirteen
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7300 Boston Boulevard, Building Thirteen
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		608
Original Building		4,773
Costs Capitalized Subsequent to Acquisition		709
Land and improvements		661
Buildings and Improvements		5,429
Land Held for Development		0
Development and Construction in Progress		0
Total		6,090
Accumulated Depreciation		$ 2,785
Year(s) Built / Renovated		2002
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7500 Boston Boulevard, Building Six
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7500 Boston Boulevard, Building Six
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		138
Original Building		3,749
Costs Capitalized Subsequent to Acquisition		1,655
Land and improvements		406
Buildings and Improvements		5,136
Land Held for Development		0
Development and Construction in Progress		0
Total		5,542
Accumulated Depreciation		$ 3,965
Year(s) Built / Renovated		1985
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
8000 Corporate Court, Building Eleven
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		8000 Corporate Court, Building Eleven
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		136
Original Building		3,071
Costs Capitalized Subsequent to Acquisition		1,596
Land and improvements		774
Buildings and Improvements		4,029
Land Held for Development		0
Development and Construction in Progress		0
Total		4,803
Accumulated Depreciation		$ 2,909
Year(s) Built / Renovated		1989
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7374 Boston Boulevard, Building Four
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7374 Boston Boulevard, Building Four
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		241
Original Building		1,605
Costs Capitalized Subsequent to Acquisition		1,913
Land and improvements		398
Buildings and Improvements		3,361
Land Held for Development		0
Development and Construction in Progress		0
Total		3,759
Accumulated Depreciation		$ 2,578
Year(s) Built / Renovated		1984
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7375 Boston Boulevard, Building Ten
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7375 Boston Boulevard, Building Ten
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		23
Original Building		2,685
Costs Capitalized Subsequent to Acquisition		938
Land and improvements		93
Buildings and Improvements		3,553
Land Held for Development		0
Development and Construction in Progress		0
Total		3,646
Accumulated Depreciation		$ 2,570
Year(s) Built / Renovated		1988
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
7451 Boston Boulevard, Building Two
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		7451 Boston Boulevard, Building Two
Type		Office
Location		Springfield, VA
Encumbrances		$ 0
Original Land		249
Original Building		1,542
Costs Capitalized Subsequent to Acquisition		1,659
Land and improvements		613
Buildings and Improvements		2,837
Land Held for Development		0
Development and Construction in Progress		0
Total		3,450
Accumulated Depreciation		$ 2,429
Year(s) Built / Renovated		1982
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
32 Hartwell Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		32 Hartwell Avenue
Type		Office
Location		Lexington, MA
Encumbrances		$ 0
Original Land		168
Original Building		1,943
Costs Capitalized Subsequent to Acquisition		1,012
Land and improvements		314
Buildings and Improvements		2,809
Land Held for Development		0
Development and Construction in Progress		0
Total		3,123
Accumulated Depreciation		$ 1,974
Year(s) Built / Renovated		1968/1979/1987
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
164 Lexington Road
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		164 Lexington Road
Type		Office
Location		Billerica, MA
Encumbrances		$ 0
Original Land		592
Original Building		1,370
Costs Capitalized Subsequent to Acquisition		319
Land and improvements		643
Buildings and Improvements		1,638
Land Held for Development		0
Development and Construction in Progress		0
Total		2,281
Accumulated Depreciation		$ 868
Year(s) Built / Renovated		1982
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
The Avant at Reston Town Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		The Avant at Reston Town Center
Type		Residential
Location		Reston, VA
Encumbrances		$ 0
Original Land		20,350
Original Building		91,995
Costs Capitalized Subsequent to Acquisition		830
Land and improvements		20,350
Buildings and Improvements		92,825
Land Held for Development		0
Development and Construction in Progress		0
Total		113,175
Accumulated Depreciation		$ 9,780
Year(s) Built / Renovated		2014
Year(s) Acquired		2010
Depreciable Lives (Years)	[3]	-1
The Lofts at Atlantic Wharf
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		The Lofts at Atlantic Wharf
Type		Residential
Location		Boston, MA
Encumbrances		$ 0
Original Land		3,529
Original Building		54,891
Costs Capitalized Subsequent to Acquisition		1,768
Land and improvements		3,529
Buildings and Improvements		56,659
Land Held for Development		0
Development and Construction in Progress		0
Total		60,188
Accumulated Depreciation		$ 9,622
Year(s) Built / Renovated		2011
Year(s) Acquired		2007
Depreciable Lives (Years)	[3]	-1
Boston Marriott Cambridge
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Boston Marriott Cambridge
Type		Hotel
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		478
Original Building		37,918
Costs Capitalized Subsequent to Acquisition		38,799
Land and improvements		1,201
Buildings and Improvements		75,994
Land Held for Development		0
Development and Construction in Progress		0
Total		77,195
Accumulated Depreciation		$ 47,753
Year(s) Built / Renovated		1986
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
Kendall Center Green Garage
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Kendall Center Green Garage
Type		Garage
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		0
Original Building		35,035
Costs Capitalized Subsequent to Acquisition		7,448
Land and improvements		103
Buildings and Improvements		42,380
Land Held for Development		0
Development and Construction in Progress		0
Total		42,483
Accumulated Depreciation		$ 10,779
Year(s) Built / Renovated		1984
Year(s) Acquired		2006
Depreciable Lives (Years)	[3]	-1
Kendall Center Yellow Garage
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Kendall Center Yellow Garage
Type		Garage
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		1,256
Original Building		15,697
Costs Capitalized Subsequent to Acquisition		1,621
Land and improvements		1,434
Buildings and Improvements		17,140
Land Held for Development		0
Development and Construction in Progress		0
Total		18,574
Accumulated Depreciation		$ 5,181
Year(s) Built / Renovated		2006
Year(s) Acquired		2004
Depreciable Lives (Years)	[3]	-1
Kendall Center Blue Garage
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Kendall Center Blue Garage
Type		Garage
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		1,163
Original Building		11,633
Costs Capitalized Subsequent to Acquisition		2,283
Land and improvements		1,579
Buildings and Improvements		13,500
Land Held for Development		0
Development and Construction in Progress		0
Total		15,079
Accumulated Depreciation		$ 9,209
Year(s) Built / Renovated		1990
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	-1
Salesforce Tower
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Salesforce Tower
Type		Development
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		947,106
Land and improvements		200,349
Buildings and Improvements		55,318
Land Held for Development		0
Development and Construction in Progress		691,439
Total		947,106
Accumulated Depreciation		$ 116
Year(s) Built / Renovated		N/A
Year(s) Acquired		2013
Depreciable Lives (Years)		N/A
Signature at Reston
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Signature at Reston
Type		Development
Location		Reston, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		203,650
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		203,650
Total		203,650
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2013
Depreciable Lives (Years)		N/A
Proto Kendall Square
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Proto Kendall Square
Type		Development
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		82,905
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		82,905
Total		82,905
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2015
Depreciable Lives (Years)		N/A
145 Broadway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		145 Broadway
Type		Development
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		121
Original Building		0
Costs Capitalized Subsequent to Acquisition		79,979
Land and improvements		324
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		79,776
Total		80,100
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		NA
Year(s) Acquired		1997
Depreciable Lives (Years)		N/A
191 Spring Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		191 Spring Street
Type		Development
Location		Lexington, MA
Encumbrances		$ 0
Original Land		2,850
Original Building		27,166
Costs Capitalized Subsequent to Acquisition		35,174
Land and improvements		3,151
Buildings and Improvements		46,288
Land Held for Development		0
Development and Construction in Progress		15,751
Total		65,190
Accumulated Depreciation		$ 19,034
Year(s) Built / Renovated		1971/1995
Year(s) Acquired		1997
Depreciable Lives (Years)	[3]	(1)
6595 Springfield Center Drive (TSA Headquarters)
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		6595 Springfield Center Drive (TSA Headquarters)
Type		Development
Location		Springfield, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		41,918
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		41,918
Total		41,918
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2007
Depreciable Lives (Years)		N/A
MacArthur Station Residences
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		MacArthur Station Residences
Type		Development
Location		Oakland, CA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		37,644
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		37,644
Total		37,644
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		N/A
Depreciable Lives (Years)		N/A
20 CityPoint
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		20 CityPoint
Type		Development
Location		Waltham, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		13,056
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		0
Development and Construction in Progress		13,056
Total		13,056
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2007
Depreciable Lives (Years)		N/A
Tower Oaks Master Plan
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Tower Oaks Master Plan
Type		Land
Location		Rockville, MD
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		29,419
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		29,419
Development and Construction in Progress		0
Total		29,419
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		1998
Depreciable Lives (Years)		N/A
Plaza at Almaden
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Plaza at Almaden
Type		Land
Location		San Jose, CA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		29,085
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		29,085
Development and Construction in Progress		0
Total		29,085
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2006
Depreciable Lives (Years)		N/A
Springfield Metro Center
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Springfield Metro Center
Type		Land
Location		Springfield, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		19,684
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		19,684
Development and Construction in Progress		0
Total		19,684
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2007
Depreciable Lives (Years)		N/A
214 Third Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		214 Third Avenue
Type		Land
Location		Waltham, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		13,815
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		13,815
Development and Construction in Progress		0
Total		13,815
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2006
Depreciable Lives (Years)		N/A
Reston Gateway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Reston Gateway
Type		Land
Location		Reston, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		13,730
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		13,730
Development and Construction in Progress		0
Total		13,730
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		1998
Depreciable Lives (Years)		N/A
103 Fourth Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		103 Fourth Avenue
Type		Land
Location		Waltham, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		12,115
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		12,115
Development and Construction in Progress		0
Total		12,115
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2007
Depreciable Lives (Years)		N/A
Crane Meadow
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Crane Meadow
Type		Land
Location		Marlborough, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		8,836
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		8,836
Development and Construction in Progress		0
Total		8,836
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2000
Depreciable Lives (Years)		N/A
Washingtonian North
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Washingtonian North
Type		Land
Location		Gaithersburg, MD
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		7,645
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		7,645
Development and Construction in Progress		0
Total		7,645
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		1998
Depreciable Lives (Years)		N/A
2100 Pennsylvania Avenue
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		2100 Pennsylvania Avenue
Type		Land
Location		Washington, DC
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		6,543
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		6,543
Development and Construction in Progress		0
Total		6,543
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		N/A
Depreciable Lives (Years)		N/A
Fourth and Harrison
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Fourth and Harrison
Type		Land
Location		San Francisco, CA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		5,659
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		5,659
Development and Construction in Progress		0
Total		5,659
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		N/A
Depreciable Lives (Years)		N/A
North First Master Plan
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		North First Master Plan
Type		Land
Location		San Jose, CA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		3,018
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		3,018
Development and Construction in Progress		0
Total		3,018
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2007
Depreciable Lives (Years)		N/A
Broad Run Business Park
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Broad Run Business Park
Type		Land
Location		Loudoun County, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		2,367
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		2,367
Development and Construction in Progress		0
Total		2,367
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		1998
Depreciable Lives (Years)		N/A
Kendall Center Master Plan
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Kendall Center Master Plan
Type		Land
Location		Cambridge, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		2,207
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		2,207
Development and Construction in Progress		0
Total		2,207
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		N/A
Depreciable Lives (Years)		N/A
Weston Quarry
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Weston Quarry
Type		Land
Location		Weston, MA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		1,194
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		1,194
Development and Construction in Progress		0
Total		1,194
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2001
Depreciable Lives (Years)		N/A
Reston Overlook Master Plan
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Property Name		Reston Overlook Master Plan
Type		Land
Location		Reston, VA
Encumbrances		$ 0
Original Land		0
Original Building		0
Costs Capitalized Subsequent to Acquisition		39
Land and improvements		0
Buildings and Improvements		0
Land Held for Development		39
Development and Construction in Progress		0
Total		39
Accumulated Depreciation		$ 0
Year(s) Built / Renovated		N/A
Year(s) Acquired		2000
Depreciable Lives (Years)		N/A
Minimum [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Life Used for Depreciation		life of the lease

[1] Includes unamortized deferred financing costs totaling approximately $(34.7) million.

[2] Includes pre-development costs.

[3] Depreciation of the buildings and improvements are calculated over lives ranging from the life of the lease to 40 years.