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, ft² in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
USD ($)
ft²
item
property
Sep. 30, 2016
USD ($)
ft²
item
property
Dec. 31, 2015
property
Real Estate Properties [Line Items]      
Unrealized losses on properties held for sale   $ (24,393)  
Number of properties sold | property     6
Sales proceeds   $ 452,183  
Multi-Family Properties [Member]      
Real Estate Properties [Line Items]      
Number of units | item 5,214 5,214  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]      
Real Estate Properties [Line Items]      
Estimated expected sales proceeds $ 113,000    
Assets held for sale, Deferred charges and other assets 7,600 $ 7,600  
Assets held for sale, Unbilled rents receivable, net 5,600 5,600  
Assets held for sale, Accounts payable, accrued expenses and other liabilities 2,900 2,900  
Assets held for sale, Rents received in advance and security deposits $ 2,800 2,800  
Expected assets to be written off   12,500  
Expected liabilities to be written off   $ 2,800  
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Office [Member]      
Real Estate Properties [Line Items]      
Area of property (in square feet) | ft² 1.8 1.8  
Number of properties held for sale | property 14 14