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Unsecured Lines of Credit Payable - Revolving Credit Facility Unused and Available Credit (Details) - USD ($)
Sep. 30, 2020
Dec. 31, 2019
Mar. 31, 2018
Line of Credit Facility [Line Items]      
Borrowings outstanding $ (186,000,000) $ (56,000,000)  
Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Committed capacity 700,000,000   $ 700,000,000.0
Borrowings outstanding (186,000,000) $ (56,000,000)  
Unused and available $ 514,000,000