Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Balance Sheets
Allowance for doubtful accounts	$ 26	$ 24
Accumulated depreciation and amortization	$ 17,826	$ 17,152
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	196,076,817	190,966,584