Annual report pursuant to Section 13 and 15(d)

Recent Transactions (Real Estate Held For Sale/Discontinued Operations/Dispositions) (Narrative) (Details)

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Recent Transactions (Real Estate Held For Sale/Discontinued Operations/Dispositions) (Narrative) (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
ft²
property
item
Dec. 31, 2018
USD ($)
ft²
item
Dec. 19, 2019
ft²
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale $ (36,225,000)    
Land and other Impairments $ 32,444,000 $ 24,566,000  
Suburban Office Portfolio [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft²     6,600,000
Number of properties sold | property 2    
Area Of Real Estate Property Sold | ft² 497,000    
Gain (loss) on sale of property $ 52,200,000    
Office [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft²     6,600,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Assets held for sale, Deferred charges and other assets 70,414,000    
Assets held for sale, Unbilled rents receivable, net 32,144,000    
Assets held for sale, Accounts payable, accrued expenses and other liabilities $ 22,817,000    
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Suburban Office Portfolio [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft² 6,100,000    
Number of properties held for sale | property 35    
Disposal Group, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale   $ (20,135,000)  
Area Of Real Estate Property Sold | ft² 4,490,578 2,405,654  
Sales proceeds $ 1,052,427,000 $ 324,050,000  
Gain (loss) on sale of property $ 382,151,000 $ 119,571,000  
Number of building sold | item 68 30  
Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale $ (137,876,000)    
Gain (loss) on sale of property 1,702,000    
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft²   845,000  
Estimated expected sales proceeds 123,100,000    
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale   $ 20,100,000  
Assets held for sale, Deferred charges and other assets   2,900,000  
Assets held for sale, Unbilled rents receivable, net   1,700,000  
Assets held for sale, Accounts payable, accrued expenses and other liabilities   2,300,000  
Expected assets to be written off $ 3,900,000    
Expected liabilities to be written off   $ 1,700,000  
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]      
Real Estate Properties [Line Items]      
Number of real estate properties | item   159  
Number of properties held for sale | property 35    
Unrealized losses on rental properties held for sale $ (174,100,000)    
Number of properties not expected to recover from estimated net sales proceeds | property 21    
Land and other Impairments $ 32,400,000    
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member] | Discontinued Operations [Member] | Office [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale (137,900,000)    
Fort Lee, Parsippany, Madison, Short Hills, Edison, Red Bank, Florham Park [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale $ 174,100,000    
Fort Lee, Parsippany, Madison, Short Hills, Edison, Red Bank, Florham Park [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Number of properties held for sale | property 21    
Assets held for sale, Deferred charges and other assets $ 68,684,000    
Assets held for sale, Unbilled rents receivable, net 30,188,000    
Assets held for sale, Accounts payable, accrued expenses and other liabilities 21,025,000    
Fort Lee, Parsippany, Madison, Short Hills, Edison, Red Bank, Florham Park [Member] | Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale 137,900,000    
Land and other Impairments $ 32,400,000