Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Disclosure Of Fair Value Of Assets And Liabilities (Narrative) (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
property
Dec. 31, 2016
USD ($)
property
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value of Company's long-term debt $ 2,764,033,000 $ 2,308,488,000
Principal balance outstanding $ 2,809,568,000 $ 2,340,009,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Number of properties held for sale | property   8
Aggregate carrying value   $ 39,700,000
Estimated expected sales proceeds   49,000,000
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  
Estimated expected sales proceeds 223,000,000  
Disposal Group, Not Discontinued Operations [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Sales proceeds 399,332,000 664,481,000
Unrealized losses on rental properties held for sale $ (12,331,000) $ (7,665,000)