Restructuring and Integration - Summary of Restructuring Costs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2021	Jun. 30, 2021
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	$ 3	$ 9
Inception to Date	103
Estimated Remaining Costs	32
Estimated Total Cost	135
Employer Solutions
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	3	8
Inception to Date	90
Estimated Remaining Costs	25
Estimated Total Cost	115
Employer Solutions | Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	1	6
Inception to Date	46
Estimated Remaining Costs	21
Estimated Total Cost	67
Employer Solutions | Other Restructuring Costs
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	2	2
Inception to Date	44
Estimated Remaining Costs	4
Estimated Total Cost	48
Professional Services
Restructuring Cost And Reserve [Line Items]
Restructuring Costs		1
Inception to Date	13
Estimated Remaining Costs	7
Estimated Total Cost	20
Professional Services | Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring Costs		$ 1
Inception to Date	8
Estimated Remaining Costs	5
Estimated Total Cost	13
Professional Services | Other Restructuring Costs
Restructuring Cost And Reserve [Line Items]
Inception to Date	5
Estimated Remaining Costs	2
Estimated Total Cost	$ 7