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
12 Months Ended
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, Including Discontinued Operation, Consideration $ 252,100
Gain on sale of real estate 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, Including Discontinued Operation, Consideration 12,100 [1]
Gain on sale of real estate 527 [1]
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, Including Discontinued Operation, Consideration 111,500 [2]
Gain on sale of real estate 23,585 [2]
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, Including Discontinued Operation, Consideration 128,500 [3]
Gain on sale of real estate $ 77,592 [3]
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 692,000 [2]
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 491,000 [3]
[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.