Goodwill and Intangible Assets, Net - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Jul. 01, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Goodwill	$ 3,356,000,000		$ 2,250,000,000	$ 3,356,000,000			$ 2,245,000,000
Non-taxable portion of goodwill acquired during period				1,500,000,000
Impairment of goodwill	0		0	0
Amortization of intangible assets	74,000,000	$ 51,000,000	100,000,000		$ 151,000,000
Accumulated amortization	74,000,000			74,000,000		$ 0	$ 672,000,000
Employer Solutions
Finite Lived Intangible Assets [Line Items]
Goodwill			$ 3,400,000,000
Professional Services
Finite Lived Intangible Assets [Line Items]
Goodwill	$ 36,000,000			$ 36,000,000