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Unsecured Lines Of Credit Payable - Revolving Line of Credit (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]      
Committed capacity $ 700,000,000 $ 600,000,000 $ 600,000,000
Borrowings outstanding (188,000,000) (166,000,000) $ (120,000,000)
credit facility 2015 [Member] | Line of Credit [Member]      
Line of Credit Facility [Line Items]      
Committed capacity   600,000,000  
credit facility 2018 [Member]      
Line of Credit Facility [Line Items]      
Committed capacity 1,500,000,000    
credit facility 2018 [Member] | Line of Credit [Member]      
Line of Credit Facility [Line Items]      
Committed capacity 700,000,000    
Borrowings outstanding (188,000,000) $ (166,000,000)  
Unused and available $ 512,000,000