Real Estate Properties - Assets Held-for-Sale (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) ft² property building	Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items]
Assets of properties held for sale	$ 408,626	$ 0
Number of Properties | property	105
Number of Buildings | building	164
Held-for-sale
Real Estate Properties [Line Items]
Assets of properties held for sale	$ 355,858
Number of Properties | property	20
Number of Buildings | building	50
Square Footage (in square feet) | ft²	3,406,012
Gross Sale Price	$ 438,500
Held-for-sale | Washington D.C.
Real Estate Properties [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in square feet) | ft²	129,035
Gross Sale Price	$ 70,000
Held-for-sale | Northern Virginia and Maryland
Real Estate Properties [Line Items]
Number of Properties | property	8
Number of Buildings | building	34
Square Footage (in square feet) | ft²	1,635,868
Gross Sale Price	$ 201,500
Held-for-sale | Southern Virginia
Real Estate Properties [Line Items]
Number of Properties | property	11
Number of Buildings | building	15
Square Footage (in square feet) | ft²	1,641,109
Gross Sale Price	$ 167,000