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Notes Payable, net - Unsecured Revolving Credit Facility (Details) - Unsecured Debt - USD ($)
 9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]    
Notes payable $ 1,975,000,000 $ 2,025,000,000
Revolving Credit Facility 2014    
Line of Credit Facility [Line Items]    
Notes payable 250,000,000 300,000,000
Remaining borrowing capacity 150,000,000 100,000,000
Total borrowing capacity $ 400,000,000 $ 400,000,000
Debt instrument, extension period (in years) 1 year  
Revolving Credit Facility 2014 | Minimum    
Line of Credit Facility [Line Items]    
Facility fee-annual rate 0.20%  
Revolving Credit Facility 2014 | Maximum    
Line of Credit Facility [Line Items]    
Facility fee-annual rate 0.35%  
Revolving Credit Facility 2014 | LIBOR | Minimum    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.15%  
Revolving Credit Facility 2014 | LIBOR | Maximum    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.85%