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Debt - Unsecured Revolving Credit Facility (Details) - Unsecured debt - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Line of Credit Facility [Line Items]    
Outstanding borrowings $ 1,925,000,000 $ 2,475,000,000
Unsecured Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Outstanding borrowings 0 75,000,000
Remaining borrowing capacity 600,000,000 525,000,000
Total borrowing capacity $ 600,000,000 $ 600,000,000
Debt instrument term 1 year  
Unsecured Revolving Credit Facility | Minimum    
Line of Credit Facility [Line Items]    
Annual facility fee rate 0.15%  
Unsecured Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR)    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.05%  
Unsecured Revolving Credit Facility | Maximum    
Line of Credit Facility [Line Items]    
Annual facility fee rate 0.30%  
Unsecured Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR)    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.50%