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Supplementary Information - Change in Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting [Abstract]      
Balance at beginning of year $ 397 $ 333 $ 275
Additions 7 123 75
Amounts written off (163) (59) (17)
Balance at end of year $ 241 $ 397 $ 333