Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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+ Details

Segment Reporting - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
Segment Reporting - Narrative (Details) $ in Millions	Sep. 30, 2021 USD ($) member	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment member	Sep. 30, 2020 USD ($) member segment	Dec. 31, 2020 segment
Segment Reporting Information [Line Items]
Number of reportable segments \| segment			3	2	2
Total revenues	$ 2,277	$ 1,930	$ 6,840	$ 6,011
Exchanges Segment
Segment Reporting Information [Line Items]
Total revenues	$ 1,434	$ 1,337	$ 4,376	$ 4,406
Revenue \| Exchanges Segment
Segment Reporting Information [Line Items]
Number of members \| member	1		1	1
Total revenues	$ 326		$ 118	$ 276
Revenue \| Exchanges Segment \| Customer Concentration Risk
Segment Reporting Information [Line Items]
Concentration risk, percentage	12.00%		12.00%	10.00%