Restructuring and Integration - Summary of Restructuring Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	$ 85
Inception to Date	85
Estimated Remaining Costs	55
Estimated Total Cost	140
Employer Solutions
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	67
Inception to Date	67
Estimated Remaining Costs	45
Estimated Total Cost	112
Employer Solutions | Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	11
Inception to Date	11
Estimated Remaining Costs	10
Estimated Total Cost	21
Employer Solutions | Other Restructuring Costs
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	56
Inception to Date	56
Estimated Remaining Costs	35
Estimated Total Cost	91
Professional Services
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	1
Inception to Date	1
Estimated Remaining Costs	3
Estimated Total Cost	4
Professional Services | Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	1
Inception to Date	1
Estimated Remaining Costs	3
Estimated Total Cost	4
Corporate
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	17
Inception to Date	17
Estimated Remaining Costs	7
Estimated Total Cost	24
Corporate | Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	15
Inception to Date	15
Estimated Remaining Costs	6
Estimated Total Cost	21
Corporate | Other Restructuring Costs
Restructuring Cost And Reserve [Line Items]
Restructuring Costs	2
Inception to Date	2
Estimated Remaining Costs	1
Estimated Total Cost	$ 3