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Supplementary Information - Allowance for Doubtful Accounts (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for doubtful accounts      
Balance at beginning of year $ 327 $ 330 $ 186
Additions 47 159  
Amounts written off (43) (23)  
Other (7) 8  
Balance at end of year $ 327 $ 330