Quarterly report pursuant to Section 13 or 15(d)

Real Estate - Held for Sale and Sold Properties (Details)

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Real Estate - Held for Sale and Sold Properties (Details)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet | ft²         1,183,000
Disposal Group, Contract Sales Price         $ 252,100
Gain on sale of real estate $ 0 $ 24,112 $ 0 $ 24,112 101,704
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
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
[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.