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Real Estate Properties - FPO Transaction (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Oct. 02, 2017
USD ($)
ft²
building
joint_venture
property
Dec. 31, 2017
USD ($)
ft²
joint_venture
property
$ / shares
shares
Dec. 31, 2016
ft²
building
property
Business Acquisition [Line Items]      
Number of properties acquired | property     3
Number of buildings acquired | building     5
Rentable area of properties (in square feet) | ft²     830,185
Number of joint ventures | joint_venture   2  
Number of properties collateralized | property   8  
Preferred units of limited partnership   $ 20,496  
Below market, acquired lease amortization period 3 years 9 months 18 days    
First Potomac Realty Trust      
Business Acquisition [Line Items]      
Number of properties acquired | property 35    
Number of buildings acquired | building 72    
Rentable area of properties (in square feet) | ft² 6,028,072 400,000  
Number of joint ventures | joint_venture 2    
Purchase price $ 1,370,888    
Cash consideration 1,175,140    
Repayment of debt assumed 483,000    
Mortgage notes payable $ 167,936    
Number of properties collateralized | property 5    
First Potomac Realty Trust      
Business Acquisition [Line Items]      
Number of joint ventures | joint_venture 2    
Number of properties owned | property 2    
Number of buildings | building 3    
First Potomac Realty Trust | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Rentable area of properties (in square feet) | ft² 443,867    
Joint Venture Property 1 | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Percentage of ownership interest 50.00%    
Joint Venture Property 2 | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Percentage of ownership interest 51.00%    
Mortgages | FPO Mortgage | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Mortgage notes payable $ 167,548    
Fair value adjustment 388    
Fair value of debt 167,936    
Mortgages | Joint Venture Mortgage | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Mortgage notes payable $ 82,000    
Number of properties collateralized | property 2    
FPO Shareholders | First Potomac Realty Trust      
Business Acquisition [Line Items]      
Cash consideration $ 651,696    
Above market lease      
Business Acquisition [Line Items]      
Weighted average amortization period, assets acquired 3 years 2 months 12 days    
Lease origination value      
Business Acquisition [Line Items]      
Weighted average amortization period, assets acquired 3 years 1 month 6 days    
Series A Cumulative Preferred Stock      
Business Acquisition [Line Items]      
Preferred stock percentage   5.50%  
Preferred stock outstanding (in shares) | shares   1,814  
Redemption price (in dollars per share) | $ / shares   $ 11.15