Disposition and Assets Held for Sale (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 15, 2020 USD ($) ft²	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) ft²	Mar. 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale					$ 154,493	$ 157,810
Gain on sale of real estate				$ 8,088	59,477	47,121
Loss recognized for sale			$ 965	$ 1,144	17,142	$ (647)
Assets held for sale			$ 74,089		$ 74,089		$ 168,412
Arlington, Virginia
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property (in square feet) | ft²							4,230
Assets held for sale							$ 168,412
Arlington, Virginia | Metropolitan Park
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property (in square feet) | ft²	2,150						2,150
Gross sales price	$ 154,952
Cash proceeds from sale	154,493
Gain on sale of real estate	$ 59,477
Assets held for sale							$ 94,517
Arlington, Virginia | Pen Place - Land Parcel
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property (in square feet) | ft²			2,080		2,080		2,080
Gross sales price								$ 149,900
Assets held for sale			$ 74,089		$ 74,089		$ 73,895
Woodglen | Landmark
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale		$ 17,800
Loss recognized for sale		$ 3,000			$ 3,000