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)
ft² in Thousands, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
ft²
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
ft²
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
apartment
Mar. 31, 2018
USD ($)
ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Gain on Sale $ 0 $ 0 $ 2,495 $ 0    
Real estate impairment $ 0 $ 5,000 1,886 $ 5,000    
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Braddock Metro Center [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Real estate impairment         $ 9,100  
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | M Street 2445 [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Real estate impairment     $ 1,900   $ 24,100  
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | M Street 2445 [Member] | Office [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Rentable Square Feet | ft²           292
Initial contracted sales price           $ 100,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2018 Properties Sold Group [Member] | Office [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Rentable Square Feet | ft² 648   648      
Contract Sales Price $ 194,600   $ 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 [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Rentable Square Feet | ft² 356   356      
Contract Sales Price $ 93,000   $ 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 [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Rentable Square Feet | ft² 292   292      
Contract Sales Price $ 101,600   $ 101,600      
Gain on Sale     $ 2,495      
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 2017 Properties Sold Group [Member] | Walker House [Member] | Multifamily [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Number of Units | apartment         212  
Contract Sales Price         $ 32,200  
Gain on Sale         $ 23,838