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Receivables (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Receivables    
Accounts receivable $ 6,786 $ 5,687
Commodity derivative and foreign currency contract receivables 102 129
Other receivables 67 117
Receivables, gross 6,955 5,933
Allowance for doubtful accounts (33) (32)
Receivables, net $ 6,922 $ 5,901