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Supplemental Data (Summary of Accounts Receivable, Net) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Supplemental Data (Millions of dollars) [Abstract]    
From customers $ 2,203 $ 2,077
Other 168 167
Less allowance for doubtful accounts and sales discounts (56) (68)
Total $ 2,315 $ 2,176