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Unsecured Line of Credit Payable - Revolving Credit Facility Unused and Available Credit (Details) - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Line of Credit Facility [Line Items]    
Borrowings outstanding $ 0 $ (42,000,000)
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Committed capacity 700,000,000  
Borrowings outstanding 0 $ (42,000,000)
Unused and available $ 700,000,000