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Short And Long-Term Debt Other Debt Facilities (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Line of Credit Facility [Line Items]    
Letter of Credit, borrowing capacity $ 125  
Unsecured Letter of Credit Facility    
Line of Credit Facility [Line Items]    
Letter of Credit, borrowing capacity 66 $ 66
Outstanding letters of credit $ 58 $ 63