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Summary of Significant Accounting Policies (Roll Forward of the Allowance For Doubtful Accounts) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]      
Allowance for doubtful accounts beginning $ (17) $ (63) $ (41)
Bad debt expense (53) (13) (26)
Write-offs, net of recoveries 15 59 4
Foreign currency adjustments (5)    
Allowance for doubtful accounts ending $ (60) $ (17) $ (63)