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Schedule of Qualifying and Valuation Accounts (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at Beginning of Year $ 4,110 $ 3,504 $ 2,438
Acquisitions and Dispositions (365) (17) 960
Charged to Costs and Expenses 2,849 3,168 3,022
Write-offs (2,821) (2,545) (2,916)
Balance at End of Year $ 3,773 $ 4,110 $ 3,504