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Debt - Schedule of Fair Value of Financial Instruments (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Carrying value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total debt $ 34,689 $ 34,392
Estimated fair value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total debt $ 39,622 $ 38,016