CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts	$ 24	$ 21
Accumulated depreciation and amortization	$ 18,050	$ 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	201,905,532	196,963,558