Real Estate Properties - Disposition Activities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft² property	Dec. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items]
Gain on sale of real estate	$ 105,131	$ 20,661	$ 0
Loss on impairment of real estate	$ 22,255	$ 8,630	$ 9,490
Discontinued Operations, Held-for-sale
Real Estate Properties [Line Items]
Number of properties | property	6
Rentable area of properties (in square feet) | ft²	734,784
Aggregate sale price of properties sold, excluding closing costs	$ 85,363
Discontinued Operations, Held-for-sale | Stafford, VA
Real Estate Properties [Line Items]
Number of properties | property	2
Rentable area of properties (in square feet) | ft²	64,656
Aggregate sale price of properties sold, excluding closing costs	$ 14,063
Discontinued Operations, Held-for-sale | Fairfax, VA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	83,130
Aggregate sale price of properties sold, excluding closing costs	$ 22,200
Discontinued Operations, Held-for-sale | Windsor, CT
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	97,256
Aggregate sale price of properties sold, excluding closing costs	$ 7,000
Discontinued Operations, Held-for-sale | Trenton, NJ
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	267,025
Aggregate sale price of properties sold, excluding closing costs	$ 30,100
Loss on impairment of real estate	$ 9,454
Discontinued Operations, Held-for-sale | Lincolnshire, IL
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	222,717
Aggregate sale price of properties sold, excluding closing costs	$ 12,000
Disposal Group, Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties | property	58
Rentable area of properties (in square feet) | ft²	6,178,461	2,200,000
Aggregate sale price of properties sold, excluding closing costs	$ 848,853	$ 320,255
Gain on sale of real estate	105,131
Loss on impairment of real estate	$ 12,801
Disposal Group, Disposed of by Sale | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		19
Rentable area of properties (in square feet) | ft²		2,174,637
Aggregate sale price of properties sold, excluding closing costs		$ 320,255
Gain on sale of real estate		20,661
Loss on impairment of real estate		$ 3,669
Disposal Group, Disposed of by Sale | Northern Virginia and Maryland
Real Estate Properties [Line Items]
Number of properties | property	34
Rentable area of properties (in square feet) | ft²	1,635,868
Aggregate sale price of properties sold, excluding closing costs	$ 198,500
Gain on sale of real estate	0
Loss on impairment of real estate	$ 732
Disposal Group, Disposed of by Sale | Washington, D.C.
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	129,035
Aggregate sale price of properties sold, excluding closing costs	$ 70,000
Gain on sale of real estate	22,075
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Buffalo, NY
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	121,711
Aggregate sale price of properties sold, excluding closing costs	$ 16,900
Gain on sale of real estate	0
Loss on impairment of real estate	$ 5,137
Disposal Group, Disposed of by Sale | Maynard, MA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	287,037
Aggregate sale price of properties sold, excluding closing costs	$ 5,000
Gain on sale of real estate	(227)
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Kapolei, HI
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	416,956
Aggregate sale price of properties sold, excluding closing costs	$ 7,100
Gain on sale of real estate	0
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Nashua, NH
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	321,800
Aggregate sale price of properties sold, excluding closing costs	$ 25,000
Gain on sale of real estate	8,401
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Arlington, TX
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	182,630
Aggregate sale price of properties sold, excluding closing costs	$ 14,900
Gain on sale of real estate	187
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Rochester, NY
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	94,800
Aggregate sale price of properties sold, excluding closing costs	$ 4,765
Gain on sale of real estate	(104)
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Hanover, PA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	502,300
Aggregate sale price of properties sold, excluding closing costs	$ 5,500
Gain on sale of real estate	(417)
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | San Antonio, TX
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	618,017
Aggregate sale price of properties sold, excluding closing costs	$ 198,000
Gain on sale of real estate	3,869
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Topeka, KS
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	143,934
Aggregate sale price of properties sold, excluding closing costs	$ 15,600
Gain on sale of real estate	36
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Falling Waters, WV
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	40,348
Aggregate sale price of properties sold, excluding closing costs	$ 650
Gain on sale of real estate	0
Loss on impairment of real estate	$ 2,179
Disposal Group, Disposed of by Sale | Columbia,SC
Real Estate Properties [Line Items]
Number of properties | property	3
Rentable area of properties (in square feet) | ft²	180,703
Aggregate sale price of properties sold, excluding closing costs	$ 10,750
Gain on sale of real estate	0
Loss on impairment of real estate	$ 3,581
Disposal Group, Disposed of by Sale | Metro DC - MD
Real Estate Properties [Line Items]
Number of properties | property	3
Rentable area of properties (in square feet) | ft²	372,605
Aggregate sale price of properties sold, excluding closing costs	$ 61,938
Gain on sale of real estate	1,177
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Phoenix,AZ
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	122,646
Aggregate sale price of properties sold, excluding closing costs	$ 12,850
Gain on sale of real estate	860
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Houston,TX
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	497,477
Aggregate sale price of properties sold, excluding closing costs	$ 130,000
Gain on sale of real estate	59,992
Loss on impairment of real estate	$ 0
Disposal Group, Disposed of by Sale | Kansas City, KS
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	170,817
Aggregate sale price of properties sold, excluding closing costs	$ 11,700
Gain on sale of real estate	0
Loss on impairment of real estate	$ 1,172
Disposal Group, Disposed of by Sale | Minneapolis, NM | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		1
Rentable area of properties (in square feet) | ft²		193,594
Aggregate sale price of properties sold, excluding closing costs		$ 20,000
Gain on sale of real estate		0
Loss on impairment of real estate		$ 640
Disposal Group, Disposed of by Sale | New York, NY | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		1
Rentable area of properties (in square feet) | ft²		187,060
Aggregate sale price of properties sold, excluding closing costs		$ 118,500
Gain on sale of real estate		17,249
Loss on impairment of real estate		$ 0
Disposal Group, Disposed of by Sale | Sacramento, CA | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		1
Rentable area of properties (in square feet) | ft²		110,500
Aggregate sale price of properties sold, excluding closing costs		$ 10,755
Gain on sale of real estate		0
Loss on impairment of real estate		$ 3,029
Disposal Group, Disposed of by Sale | Golden, CO | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		1
Rentable area of properties (in square feet) | ft²		43,231
Aggregate sale price of properties sold, excluding closing costs		$ 4,000
Gain on sale of real estate		54
Loss on impairment of real estate		$ 0
Disposal Group, Disposed of by Sale | Southern Virginia | One Property | Office Property
Real Estate Properties [Line Items]
Number of properties | property		15
Rentable area of properties (in square feet) | ft²		1,640,252
Aggregate sale price of properties sold, excluding closing costs		$ 167,000
Gain on sale of real estate		3,358
Loss on impairment of real estate		$ 0
July 2019 | Disposal Group, Disposed of by Sale | San Jose, CA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	71,750
Aggregate sale price of properties sold, excluding closing costs	$ 14,000
Gain on sale of real estate	(270)
Loss on impairment of real estate	$ 0
September 2019 | Disposal Group, Disposed of by Sale | San Diego, CA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	43,918
Aggregate sale price of properties sold, excluding closing costs	$ 8,950
Gain on sale of real estate	3,062
Loss on impairment of real estate	$ 0
December 2019 | Disposal Group, Disposed of by Sale | San Jose, CA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	75,621
Aggregate sale price of properties sold, excluding closing costs	$ 13,000
Gain on sale of real estate	(333)
Loss on impairment of real estate	$ 0
December 2019 | Disposal Group, Disposed of by Sale | San Diego, CA
Real Estate Properties [Line Items]
Number of properties | property	1
Rentable area of properties (in square feet) | ft²	148,488
Aggregate sale price of properties sold, excluding closing costs	$ 23,750
Gain on sale of real estate	6,823
Loss on impairment of real estate	$ 0