Quarterly report pursuant to Section 13 or 15(d)

Recent Transactions (Rental Property Held for Sale, Net) (Narrative) (Details)

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Recent Transactions (Rental Property Held for Sale, Net) (Narrative) (Details) - Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
$ in Thousands
 6 Months Ended
Jun. 30, 2018
USD ($)
ft²
property
Office [Member]  
Real Estate Properties [Line Items]  
Unrealized losses on rental properties held for sale $ 500
Paramus, Hamilton And Rochelle Park, New Jersey And White Plains, New York [Member]  
Real Estate Properties [Line Items]  
Unrealized losses on properties held for sale 500
Assets held for sale, Deferred charges and other assets 3,400
Assets held for sale, Unbilled rents receivable, net 1,100
Assets held for sale, Accounts payable, accrued expenses and other liabilities 2,100
Expected assets to be written off 4,100
Expected liabilities to be written off $ 1,100
Paramus, Hamilton And Rochelle Park, New Jersey And White Plains, New York [Member] | Office [Member]  
Real Estate Properties [Line Items]  
Area of property (in square feet) | ft² 629,000
Number of properties held for sale | property 5
Unrealized losses on properties held for sale $ 500
Estimated expected sales proceeds $ 70,500