Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current Assets:
Allowance for uncollectible accounts	$ 89	$ 13
Allowance for obsolescence	188	82
Noncurrent Assets:
Accumulated depreciation and amortization	17,511	17,027
Accumulated amortization	$ 883	$ 873
Stockholders' Equity:
Common stock, par value (usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (shares)	1,500,000,000	1,500,000,000
Common stock, shares issued (shares)	647,352,203	651,731,443
Treasury stock, at cost (shares)	9,169,683	8,959,730