Real Estate - Held for Sale and Sold Properties (Details) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate impairment		$ 5,000	$ 0	$ 5,000	$ 0
Rentable Square Feet | ft²						1,183,000
Disposal Group, Contract Sales Price						$ 252,100
Gain on sale of real estate		0	$ 77,592	0	$ 101,704	101,704
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Dulles Station II [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Contract Sales Price	[1]					12,100
Gain on sale of real estate	[1]					527
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction I [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Contract Sales Price	[2]					111,500
Gain on sale of real estate	[2]					23,585
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction II [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Contract Sales Price	[3]					128,500
Gain on sale of real estate	[3]					$ 77,592
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Walker House [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Contract Sales Price		$ 32,200		$ 32,200
Office [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction I [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²	[2]					692,000
Office [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Maryland Office Portfolio Transaction II [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²	[3]					491,000
Office [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Braddock Metro Center [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²		356,000		356,000

[1] Land held for future development and an interest in a parking garage.

[2] Maryland Office Portfolio Transaction I consists of 6110 Executive Boulevard, 600 Jefferson Plaza, Wayne Plaza and West Gude Drive.

[3] Maryland Office Portfolio Transaction II consists of 51 Monroe Street and One Central Plaza.