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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Balance at January 1, 2020 $ 43  
Provision for credit losses 24 $ 26
Write-offs (32)  
Recoveries collected 4  
Balance at March 31, 2020 $ 39