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Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Current Assets:    
Allowance for uncollectible accounts $ 80 $ 89
Allowance for obsolescence 178 188
Noncurrent Assets:    
Accumulated depreciation and amortization 18,477 17,511
Accumulated amortization $ 888 $ 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) 649,637,387 647,352,203
Treasury stock, at cost (shares) 9,722,509 9,169,683