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Debt (Estimated Fair Value of Significant Financial Instruments) (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt $ 516 $ 517
Long-term debt 10,257 10,796
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Short-term debt 523 523
Long-term debt $ 11,853 $ 12,858