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Summary of Significant Accounting Policies (Tables)
9 Months Ended
Sep. 30, 2021
Accounting Policies [Abstract]  
Allowance for doubtful accounts receivable
The following table provides a rollforward of changes in the allowance for doubtful accounts:
Nine Months Ended
September 30,
(In millions)20212020
Beginning balance$$10 
Provision
Recoveries— (3)
  Write-offs charged against the allowance— (4)
Ending balance$$