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Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Statement of Financial Position [Abstract]    
Allowance for doubtful accounts $ 20 $ 25
Accumulated depreciation and amortization $ 17,003 $ 16,420
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 188,284,965 183,105,326