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Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Current Assets:    
Allowance for uncollectible accounts $ 16 $ 13
Allowance for obsolescence 90 82
Noncurrent Assets:    
Accumulated depreciation and amortization 17,506 17,027
Accumulated amortization $ 875 $ 873
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) 647,386,115 651,731,443
Treasury stock, at cost (shares) 9,549,909 8,959,730