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Significant Acquisitions and Dispositions (Summary of Significant Assumptions Utilized in Estimates) (Details)
12 Months Ended
Dec. 31, 2016
$ / sqft
months
Dec. 31, 2015
$ / sqft
months
Dec. 31, 2014
$ / sqft
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot     12.50
Minimum [Member]      
Fair Value Inputs, Discount Rate 7.46% 7.07%  
Fair Value Inputs, Cap Rate 6.25% 5.57%  
Lease-up period (months) | months 12 12  
Maximum [Member]      
Fair Value Inputs, Discount Rate 8.10% 7.07%  
Fair Value Inputs, Cap Rate 6.96% 5.57%  
Lease-up period (months) | months 12 12  
Duke/Quantico LLC [Member] | Minimum [Member]      
Fair Value Inputs, Discount Rate 8.00%    
Fair Value Inputs, Cap Rate 6.50%    
Lease-up period (months) | months 12    
Duke/Quantico LLC [Member] | Maximum [Member]      
Fair Value Inputs, Discount Rate 10.50%    
Fair Value Inputs, Cap Rate 9.00%    
Lease-up period (months) | months 36    
Office [Member] | Duke/Quantico LLC [Member] | Minimum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 9.34    
Office [Member] | Duke/Quantico LLC [Member] | Maximum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 18.54    
Industrial [Member] | Minimum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 3.35 4.85  
Industrial [Member] | Maximum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 3.39 4.85  
Industrial [Member] | Duke/Quantico LLC [Member] | Minimum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 8.20    
Industrial [Member] | Duke/Quantico LLC [Member] | Maximum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 8.50    
Medical Office [Member] | Minimum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 15.40 0.00  
Medical Office [Member] | Maximum [Member]      
Fair Value Measurement With Unobservable Inputs Rental Rate Per Square Foot 15.40 0.00