Accounts Receivable - Changes in Allowance for Fee-for-Service Commissions (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2020	$ 20
Balance as of June 30, 2021	19
Fee-for-Service Commission
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2020	18
Current period provision for expected credit losses	4
Write-offs charged against the allowance	(5)
Balance as of June 30, 2021	$ 17