Quarterly report pursuant to Section 13 or 15(d)

Recent Transactions (Rental Properties Held for Sale) (Narrative) (Details)

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Recent Transactions (Rental Properties Held for Sale) (Narrative) (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
ft²
property
Dec. 31, 2015
USD ($)
property
Real Estate Properties [Line Items]    
Aggregate carrying value $ 36,110,000  
Mortgage loan 767,573,000 $ 726,611,000
Number of properties sold | property   6
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]    
Real Estate Properties [Line Items]    
Assets held for sale, Deferred charges and other assets 13,200,000  
Assets held for sale, Unbilled rents receivable, net 20,100,000  
Assets held for sale, Accounts receivable, net of allowance 1,300,000  
Assets held for sale, Accounts payable, accrued expenses and other liabilities 3,400,000  
Assets held for sale, Rents received in advance and security deposits 1,200,000  
Expected assets to be written off 31,300,000  
Expected liabilities to be written off $ 796,000  
1400L Street, NW in Washington, D.C. and 125 Broad Street in New York, New York [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]    
Real Estate Properties [Line Items]    
Area of property (in square feet) | ft² 683,000  
Number of properties held for sale | property 2  
Estimated expected sales proceeds $ 272,000,000  
Expected disposition completion date second quarter of 2016