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Investments in Real Estate (Held for Sale) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
property
Dec. 31, 2016
USD ($)
property
Dec. 31, 2015
USD ($)
Business Acquisition [Line Items]      
Number of real estate properties | property 205 187  
Assets $ 21,404,345,000 $ 12,192,585,000  
Liabilities 10,300,993,000 7,060,288,000  
Impairment charges, real estate $ 29,000,000 $ 0 $ 0
United States [Member]      
Business Acquisition [Line Items]      
Number of real estate properties | property 3    
Held-for-sale      
Business Acquisition [Line Items]      
Number of properties, classified as held for sale | property 8 3  
Number of real estate properties | property 8    
Assets $ 139,500,000 $ 56,100,000  
Liabilities $ 5,000,000 $ 2,600,000