Annual report pursuant to Section 13 and 15(d)

Disclosure Of Fair Value Of Assets And Liabilities (Narrative) (Details)

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Disclosure Of Fair Value Of Assets And Liabilities (Narrative) (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
property
Dec. 31, 2018
USD ($)
ft²
property
item
Dec. 19, 2019
ft²
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Fair value of Company's long-term debt $ 2,791,629,000 $ 2,711,712,000  
Principal balance outstanding 2,808,517,000 2,792,651,000  
Unrealized losses on rental properties held for sale (36,225,000)    
Land and other Impairments 32,444,000 $ 24,566,000  
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Estimated expected sales proceeds $ 123,100,000    
Area of property (in square feet) | ft²   845,000  
Conshohocken And Bala Cynwyd, Pennsylvania [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Number of real estate properties | property 2    
Land and other Impairments $ 2,700,000    
Office [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Area of property (in square feet) | ft²     6,600,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Unrealized losses on rental properties held for sale   $ 20,100,000  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Conshohocken And Bala Cynwyd, Pennsylvania [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Land and other Impairments   $ 24,600,000  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member] | Parsippany, Paramus, Rochelle Park, Hamilton And Wall, New Jersey And White Plains, New York [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Number of properties held for sale | property   6  
Estimated expected sales proceeds   $ 124,000,000  
Number of properties not expected to recover from estimated net sales proceeds | property   4  
Unrealized losses on rental properties held for sale   $ 20,100,000  
Number of real estate properties | item   159  
Properties aggregate net book value   $ 108,800,000  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member] | Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Number of properties held for sale | property 35    
Number of properties not expected to recover from estimated net sales proceeds | property 21    
Unrealized losses on rental properties held for sale $ (174,100,000)    
Number of real estate properties | item   159  
Land and other Impairments 32,400,000    
Properties aggregate net book value 966,000,000    
Disposal Group, Not Discontinued Operations [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Unrealized losses on rental properties held for sale   $ (20,135,000)  
Discontinued Operations [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Unrealized losses on rental properties held for sale (137,876,000)    
Discontinued Operations [Member] | Office [Member] | Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Unrealized losses on rental properties held for sale $ (137,900,000)