Annual report pursuant to Section 13 and 15(d)

Recent Transactions (Dispositions/Rental Property Held for Sale) (Narrative) (Details)

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Recent Transactions (Dispositions/Rental Property Held for Sale) (Narrative) (Details)
$ in Thousands
1 Months Ended 12 Months Ended
Jan. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
ft²
property
item
Dec. 31, 2017
USD ($)
ft²
property
item
Disposal Group, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale   $ (20,135) $ (12,331)
Sales proceeds   $ 324,050 $ 399,332
Number of properties sold | item   30 60
Parsippany, Paramus, Rochelle Park, Hamilton And Wall, New Jersey And White Plains, New York [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Assets held for sale, Deferred charges and other assets     $ 9,800
Assets held for sale, Unbilled rents receivable, net     4,700
Assets held for sale, Accounts payable, accrued expenses and other liabilities     4,600
Expected assets to be written off     13,400
Expected liabilities to be written off     $ 300
Parsippany, Paramus, Rochelle Park, Hamilton And Wall, New Jersey And White Plains, New York [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft²     2,000,000
Number of properties held for sale | property     21
Unrealized losses on rental properties held for sale     $ 12,300
Estimated expected sales proceeds     223,000
Sales proceeds     $ 12,300
Number of properties not expected to recover from estimated net sales proceeds | property     7
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member] | Office [Member]      
Real Estate Properties [Line Items]      
Unrealized losses on rental properties held for sale   $ 20,100  
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]      
Assets held for sale, Deferred charges and other assets   2,900  
Assets held for sale, Unbilled rents receivable, net   1,700  
Assets held for sale, Accounts payable, accrued expenses and other liabilities   2,300  
Expected assets to be written off   3,900  
Expected liabilities to be written off   $ 1,700  
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  
Area of property (in square feet) | ft²   845,000  
Number of properties held for sale | property   6  
Unrealized losses on rental properties held for sale   $ 20,100  
Estimated expected sales proceeds   $ 124,000  
Number of properties not expected to recover from estimated net sales proceeds | property   4  
Fort Lee, Newark, Paramus, Bridgewater, Morris Plains And Rahway, New Jersey [Member] | Disposal Group, Not Discontinued Operations [Member] | Office [Member] | Subsequent Event [Member]      
Real Estate Properties [Line Items]      
Sales proceeds $ 54,500    
Number of properties sold | item 3