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Debt - Debt Covenants (Details)
$ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]      
Coverage ratio of debt-to-EBITDA, maximum 5.00    
Ratio of debt-to-EBITDA during acquisition period, maximum 5.50    
Threshold for acquisition period qualification $ 150    
Outstanding letters of credit 129 $ 157  
Credit agreements and commercial paper program      
Debt Instrument [Line Items]      
Total borrowings 29,300 13,300 $ 45,400
Total repayments $ 29,000 $ 12,900 $ 46,300
Letters of credit | Maximum      
Debt Instrument [Line Items]      
Periods for which letters of credit are issued 70 days