Annual report pursuant to Section 13 and 15(d)

Unsecured Lines of Credit Payable - Revolving Credit Facility Unused and Available Credit (Details)

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Unsecured Lines of Credit Payable - Revolving Credit Facility Unused and Available Credit (Details) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]        
Committed capacity $ 700,000,000 $ 700,000,000   $ 600,000,000
Borrowings outstanding (56,000,000) (188,000,000)   (166,000,000)
Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Committed capacity 700,000,000   $ 700,000,000.0 $ 600,000,000.0
Borrowings outstanding (56,000,000) $ (188,000,000)    
Unused and available $ 644,000,000