Accounts Receivable - Changes in Allowance for Fee-for-Service Commissions (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2020	$ 20
Balance as of March 31, 2021	17
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	1
Write-offs charged against the allowance	(5)
Balance as of March 31, 2021	$ 14