Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
Disclosure Of Fair Value Of Assets And Liabilities (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
property
Sep. 30, 2018
USD ($)
property
Dec. 31, 2017
USD ($)
property
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Fair value of Company's long-term debt $ 2,577,049,000 $ 2,577,049,000 $ 2,764,033,000
Principal balance outstanding 2,807,718,000 2,807,718,000 2,809,568,000
Aggregate carrying value 83,152,000 83,152,000 $ 171,578,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Paramus, Bridgewater, Rochelle Park, Morris Plains and Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Aggregate carrying value 83,152,000 83,152,000  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Unrealized losses on rental properties held for sale $ 19,600,000 $ 20,100,000  
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     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 Measurement Inputs and Valuation Techniques [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, Bridgewater, Rochelle Park, Morris Plains and Rahway, New Jersey [Member]      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Number of properties held for sale | property 4 4  
Aggregate carrying value $ 83,200,000 $ 83,200,000  
Estimated expected sales proceeds   85,500,000  
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)