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Accounts Receivable Accounts Receivable (Activity in the Allowance for Doubtful Accounts) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance beginning of year $ 97 $ 86 $ 91
Additions- charged to expense 72 107 82
Deductions- write-offs (78) (96) (87)
Balance end of year $ 91 $ 97 $ 86