Quarterly report pursuant to Section 13 or 15(d)

Properties, Net (Details 4)

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Properties, Net (Details 4)
 9 Months Ended
Sep. 30, 2017
USD ($)
ft²
Property
Oct. 27, 2017
USD ($)
ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Sale price of land $ 14,300,000  
Gain on sale of land $ 4,200,000  
Operating Properties | Defense/Information Technology Locations | Data Center Shells | 201 Technology Drive | Subsequent Event    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Square footage of real estate properties (in square feet) | ft²   103,000
Transaction value   $ 29,200,000
Operating Properties | Defense/Information Technology Locations | Data Center Shells | 11751 Meadowville Lane | Subsequent Event    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Square footage of real estate properties (in square feet) | ft²   193,000
Transaction value   $ 44,300,000
Indemnification amount   $ 20,000,000
Dispositions | Operating Properties    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Number of real estate properties | Property 10  
Square footage of real estate properties (in square feet) | ft² 639,000  
Transaction value $ 88,800,000  
Gain on Sale $ 1,180,000  
Dispositions | Operating Properties | Defense/Information Technology Locations | Northern Virginia Defense/IT | 3120 Fairview Park Drive    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Number of real estate properties | Property 1  
Square footage of real estate properties (in square feet) | ft² 190,000  
Transaction value $ 39,000,000  
Gain on Sale $ 0  
Dispositions | Operating Properties | Defense/Information Technology Locations | Fort Meade/BW Corridor | 1334 Ashton Road    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Number of real estate properties | Property 1  
Square footage of real estate properties (in square feet) | ft² 37,000  
Transaction value $ 2,300,000  
Gain on Sale $ 0  
Dispositions | Operating Properties | Regional Office and Other [Member] | White Marsh, Maryland    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Number of real estate properties | Property 8  
Square footage of real estate properties (in square feet) | ft² 412,000  
Transaction value $ 47,500,000  
Gain on Sale $ 1,180,000