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Debt - Maturities and Covenants (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]      
Weighted-average maturity of long-term debt 12 years    
Outstanding letters of credit $ 166 $ 73  
Maturities      
2018 247    
2019 500    
2020 500    
2021 600    
2022 750    
Thereafter $ 6,653    
Credit agreements      
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    
Credit agreements and commercial paper program      
Debt Instrument [Line Items]      
Total borrowings 60,800 60,300 $ 62,200
Total repayments $ 61,500 $ 61,000 $ 61,300
Letters of credit | Maximum      
Debt Instrument [Line Items]      
Periods for which letters of credit are issued 70 days