Accounts Receivable - Changes in Allowance for Fee-for-Service Commissions (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance as of December 31, 2019	$ 21
Balance at June 30, 2020	17
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	4
Write-offs charged against the allowance	(8)
Balance at June 30, 2020	$ 15