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Debt - Unsecured Revolving Credit Facility (Details) - Unsecured Debt - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Outstanding borrowings $ 1,985,000,000 $ 1,975,000,000
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Outstanding borrowings 110,000,000 100,000,000
Remaining borrowing capacity 490,000,000 300,000,000
Total borrowing capacity $ 600,000,000 $ 400,000,000
Revolving Credit Facility | Minimum    
Line of Credit Facility [Line Items]    
Annual facility fee rate 0.15% 0.20%
Revolving Credit Facility | Maximum    
Line of Credit Facility [Line Items]    
Annual facility fee rate 0.30% 0.35%
Revolving Credit Facility | LIBOR | Minimum    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.05% 1.15%
Revolving Credit Facility | LIBOR | Maximum    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.50% 1.85%