Real Estate - Held for Sale and Sold Properties (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) ft²	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Apr. 22, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate impairment	$ 8,374	$ 1,886
M Street 2445 [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate impairment			$ 1,900	$ 24,100
Quantico Corporate Center [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²	272,000
Real estate impairment	$ 8,400
Quantico Corporate Center [Member] | Office Building [Member] | Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract Sales Price					$ 33,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | M Street 2445 [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Initial contracted sales price		100,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2018 Properties Sold Group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contract Sales Price		$ 101,600
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2018 Properties Sold Group [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			648,000
Contract Sales Price			$ 194,600
Gain on Sale			$ 2,495
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2018 Properties Sold Group [Member] | Braddock Metro Center [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			356,000
Contract Sales Price			$ 93,000
Gain on Sale			$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2018 Properties Sold Group [Member] | M Street 2445 [Member] | Office Building [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²			292,000