Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current Assets:
Allowance for uncollectible accounts	$ 78	$ 89
Allowance for obsolescence	174	188
Noncurrent Assets:
Accumulated depreciation and amortization	18,693	17,511
Accumulated amortization	$ 890	$ 883
Stockholders' Equity:
Common stock, par value (USD per share)	$ 0.0001	$ 0.0001
Common stock, authorized (shares)	1,500,000,000	1,500,000,000
Common stock, issued (shares)	649,758,099	647,352,203
Treasury stock, at cost (shares)	9,744,281	9,169,683