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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2020
Sep. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance $ 47 $ 43  
Provision for credit losses 26 74 $ 87
Write-offs (19) (71)  
Recoveries collected 3 11  
Ending balance $ 57 $ 57