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CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
CONSOLIDATED BALANCE SHEETS    
Allowance for doubtful accounts $ 29 $ 21
Accumulated depreciation and amortization $ 18,264 $ 17,704
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized 1,500,000,000 1,500,000,000
Common stock, shares issued 630,282,461 630,282,461
Treasury stock, shares 206,299,352 196,963,558