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 2 Months Ended 12 Months Ended
Feb. 28, 2018
USD ($)
Feb. 28, 2017
USD ($)
Dec. 31, 2017
USD ($)
ft²
item
property
Dec. 31, 2016
USD ($)
ft²
item
property
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]        
Real Estate Properties [Line Items]        
Assets held for sale, Deferred charges and other assets       $ 1,700
Assets held for sale, Unbilled rents receivable, net       1,200
Assets held for sale, Accounts payable, accrued expenses and other liabilities       1,100
Assets held for sale, Rents received in advance and security deposits       1,900
Expected assets to be written off       2,900
Expected liabilities to be written off       $ 500
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Three Office Properties Disposed [Member]        
Real Estate Properties [Line Items]        
Sales proceeds   $ 45,800    
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]        
Real Estate Properties [Line Items]        
Area of property (in square feet) | ft²       750,000
Number of properties held for sale | property       8
Estimated expected sales proceeds       $ 49,000
Disposal Group, Not Discontinued Operations [Member]        
Real Estate Properties [Line Items]        
Unrealized losses on rental properties held for sale     $ (12,331) (7,665)
Sales proceeds     $ 399,332 $ 664,481
Number of properties sold | item     60 30
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  
Estimated expected sales proceeds     $ 223,000  
Number of properties not epected to recover from estimated net sales proceeds | property     7  
Parsippany, Paramus, Rochelle Park, Hamilton And Wall, New Jersey And White Plains, New York [Member] | Disposal Group, Not Discontinued Operations [Member] | Office [Member] | Subsequent Event [Member]        
Real Estate Properties [Line Items]        
Sales proceeds $ 25,900