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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Beginning balance $ 4 $ 10
Provision 1 2
Recoveries 0 (3)
Write-offs charged against the allowance 0 (4)
Ending balance $ 5 $ 5