Accounts Receivable - Changes in Allowance for Fee-for-Service Commissions (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2019	$ 21
Balance at September 30, 2020	18
Fee-for-Service Commission
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2019	19
Current period provision for expected credit losses	5
Write-offs charged against the allowance	(8)
Balance at September 30, 2020	$ 16