Goodwill and Intangible Assets, Net - Summary of Changes in Net Carrying Amount of Goodwill (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($)
Goodwill [Line Items]
Balance as of December 31, 2020	$ 2,245
Measurement period adjustments	2
Foreign currency translation	3
Balance as of June 30, 2021	2,250
Employer Solutions
Goodwill [Line Items]
Balance as of December 31, 2020	1,985
Measurement period adjustments	2
Foreign currency translation	2
Balance as of June 30, 2021	1,989
Professional Services
Goodwill [Line Items]
Balance as of December 31, 2020	260
Foreign currency translation	1
Balance as of June 30, 2021	$ 261