Quarterly report pursuant to Section 13 or 15(d)

Properties, Net (Details 6)

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Properties, Net (Details 6)
ft² in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Aug. 28, 2015
USD ($)
ft²
Property
Jul. 27, 2015
USD ($)
ft²
Sep. 30, 2015
USD ($)
ft²
Property
Sep. 30, 2014
USD ($)
Sep. 30, 2015
USD ($)
ft²
Property
Sep. 30, 2014
USD ($)
Construction and Redevelopment Activities            
Gain (loss) on early extinguishment of debt | $     $ 85,745 $ (176) $ 85,677 $ (446)
Newly constructed properties placed in service            
Construction and Redevelopment Activities            
Square feet of properties placed in service | ft²         897  
Number of real estate properties     7   7  
Properties under construction or contractually committed for construction            
Construction and Redevelopment Activities            
Square footage of real estate properties (in square feet) | ft²     1,000   1,000  
Number of real estate properties     6   6  
Number of partially operational real estate properties     1   1  
Properties under or approved for redevelopment            
Construction and Redevelopment Activities            
Square footage of real estate properties (in square feet) | ft²     156   156  
Number of real estate properties     4   4  
Northern Virginia | Properties under construction or contractually committed for construction            
Construction and Redevelopment Activities            
Number of real estate properties     4   4  
Greater Philadelphia and St. Mary's County, Maryland | Newly redeveloped properties placed in service            
Construction and Redevelopment Activities            
Square feet of properties placed in service | ft²         170  
Number of real estate properties     2   2  
Baltimore and Washington Corridor | Properties under construction or contractually committed for construction            
Construction and Redevelopment Activities            
Number of real estate properties     2   2  
Operating properties, net | 1550 Westbranch Drive | Northern Virginia            
Construction and Redevelopment Activities            
Square footage of real estate properties (in square feet) | ft²   160        
Transaction value of operating property disposition | $   $ 27,800        
Operating properties, net | 15000 and 15010 Conference Center Drive | Northern Virginia            
Construction and Redevelopment Activities            
Square footage of real estate properties (in square feet) | ft² 665          
Number of properties disposed 2          
Debt settled through the surrender of real estate assets | $ $ 150,000          
Fair value of real estate disposed in settlement of debt | $ 99,000          
Gain (loss) on early extinguishment of debt | $ $ 84,800          
Non-operating properties            
Construction and Redevelopment Activities            
Sale price of land | $         $ 18,100  
Gain on sale | $         $ 4,000