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)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Jul. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
ft²
item
property
Jun. 30, 2016
USD ($)
ft²
item
property
Dec. 31, 2015
property
Real Estate Properties [Line Items]        
Unrealized losses on properties held for sale     $ (3,615)  
Number of properties sold | property       6
Sales proceeds     $ 359,515  
Multi-Family Properties [Member]        
Real Estate Properties [Line Items]        
Number of units | item   5,434 5,434  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]        
Real Estate Properties [Line Items]        
Estimated expected sales proceeds   $ 84,000    
Expected disposition completion date     third quarter of 2016  
Assets held for sale, Deferred charges and other assets   2,000 $ 2,000  
Assets held for sale, Unbilled rents receivable, net   1,500 1,500  
Assets held for sale, Accounts payable, accrued expenses and other liabilities   900 900  
Assets held for sale, Rents received in advance and security deposits   $ 1,200 1,200  
Expected assets to be written off     3,500  
Expected liabilities to be written off     $ 1,200  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Four Office Properties Disposed [Member] | Subsequent Event [Member]        
Real Estate Properties [Line Items]        
Sales proceeds $ 24,900      
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]        
Real Estate Properties [Line Items]        
Area of property (in square feet) | ft²   567,000 567,000  
Number of properties held for sale | property   5 5  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Multi-Family Properties [Member]        
Real Estate Properties [Line Items]        
Number of properties held for sale | property   1 1  
Number of units | item   220 220