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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2020
Jun. 30, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance $ 39 $ 43  
Provision for credit losses 24 48 $ 52
Write-offs (20) (52)  
Recoveries collected 4 8  
Ending balance $ 47 $ 47