Real Estate Properties - Disposition Activities (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) ft² property	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) ft² property	Sep. 30, 2018 USD ($)
Real Estate Properties [Line Items]
Loss on impairment of real estate	$ 8,521	$ 0	$ 14,105	$ 5,800
Gain (loss) on sale of real estate	$ 11,463	$ 0	$ 33,538	$ 17,329
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Properties | property	47		47
Rentable area of properties (in square feet) | ft²	4,610,104		4,610,104
Aggregate sale price of properties sold, excluding closing costs			$ 584,865
Disposed of by Sale | Northern Virginia and Maryland
Real Estate Properties [Line Items]
Number of Properties | property	34		34
Rentable area of properties (in square feet) | ft²	1,635,868		1,635,868
Aggregate sale price of properties sold, excluding closing costs			$ 198,500
Loss on impairment of real estate			$ 447
Disposed of by Sale | Washington, D.C.
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	129,035		129,035
Aggregate sale price of properties sold, excluding closing costs			$ 70,000
Gain (loss) on sale of real estate			$ 22,075
Disposed of by Sale | Buffalo, NY
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	121,711		121,711
Aggregate sale price of properties sold, excluding closing costs			$ 16,900
Loss on impairment of real estate			$ 5,137
Disposed of by Sale | Maynard, MA
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	287,037		287,037
Aggregate sale price of properties sold, excluding closing costs			$ 5,000
Disposed of by Sale | Kapolei, HI
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	416,956		416,956
Aggregate sale price of properties sold, excluding closing costs			$ 7,100
Disposed of by Sale | San Jose, CA
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	71,750		71,750
Aggregate sale price of properties sold, excluding closing costs			$ 14,000
Disposed of by Sale | Nashua, NH
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	321,800		321,800
Aggregate sale price of properties sold, excluding closing costs			$ 25,000
Gain (loss) on sale of real estate			$ 8,401
Disposed of by Sale | Arlington, TX
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	182,630		182,630
Aggregate sale price of properties sold, excluding closing costs			$ 14,900
Disposed of by Sale | Rochester, NY
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	94,800		94,800
Aggregate sale price of properties sold, excluding closing costs			$ 4,765
Disposed of by Sale | Hanover, PA
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	502,300		502,300
Aggregate sale price of properties sold, excluding closing costs			$ 5,500
Disposed of by Sale | San Antonio, TX
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	618,017		618,017
Aggregate sale price of properties sold, excluding closing costs			$ 198,000
Disposed of by Sale | Topeka, KS
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	143,934		143,934
Aggregate sale price of properties sold, excluding closing costs			$ 15,600
Disposed of by Sale | Falling Waters, WV
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	40,348		40,348
Aggregate sale price of properties sold, excluding closing costs			$ 650
Loss on impairment of real estate			$ 2,179
Disposed of by Sale | San Diego, CA
Real Estate Properties [Line Items]
Number of Properties | property	1		1
Rentable area of properties (in square feet) | ft²	43,918		43,918
Aggregate sale price of properties sold, excluding closing costs			$ 8,950
Gain (loss) on sale of real estate			$ 3,062