Disposition and Assets Held for Sale (Details) $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 15, 2020 USD ($) ft²	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2019 USD ($)	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	$ 117,676
Gain on sale of real estate					59,477	39,033
Loss recognized for sale			$ 13,485	$ 1,810	16,177	$ (1,791)
Assets held for sale		$ 73,876	$ 73,876		$ 73,876		$ 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		2,080
Gross sales price								$ 149,900
Assets held for sale		$ 73,876	$ 73,876		$ 73,876		$ 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		$ 3,000