Quarterly report pursuant to Section 13 or 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)
 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
property
Dec. 31, 2017
USD ($)
property
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of Company's long-term debt $ 2,555,280,000 $ 2,764,033,000
Principal balance outstanding 2,646,437,000 2,809,568,000
Aggregate carrying value 56,419,000 $ 171,578,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Paramus, Hamilton And Rochelle Park, New Jersey And White Plains, New York [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Aggregate carrying value 56,419,000  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Unrealized losses on rental properties held for sale $ 500,000  
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 Inputs, Assets, Quantitative Information [Line Items]    
Number of properties held for sale | property   21
Aggregate carrying value   $ 171,600,000
Unrealized losses on rental properties held for sale   $ 12,300,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member] | Paramus, Rochelle Park, Hamilton And Wall, New Jersey And White Plains, New York [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Number of properties not expected to recover from estimated net sales proceeds | property   7
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member] | Paramus, Hamilton And Rochelle Park, New Jersey And White Plains, New York [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Number of properties held for sale | property 5  
Aggregate carrying value $ 56,400,000  
Estimated expected sales proceeds 70,500,000  
Disposal Group, Not Discontinued Operations [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Sales proceeds 224,559,000  
Unrealized losses on rental properties held for sale $ (500,000)