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Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Current Assets:    
Allowance for uncollectible accounts $ 75 $ 89
Allowance for obsolescence 176 188
Noncurrent Assets:    
Accumulated depreciation and amortization 17,922 17,511
Accumulated amortization $ 885 $ 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) 649,361,047 647,352,203
Treasury stock, at cost (shares) 9,699,660 9,169,683