Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Oct. 01, 2019	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Unbilled	$ 754.0		$ 754.0		$ 669.0
Disaggregation of Revenue [Line Items]
Revenues	1,561.0	$ 1,551.0	4,968.0	$ 4,906.0
Revenue Composition [Abstract]
Gross billings	10,291.0	10,067.0	32,135.0	31,763.0
Worksite employee payroll cost	8,730.0	8,516.0	27,167.0	26,857.0
Revenues	$ 1,561.0	1,551.0	$ 4,968.0	4,906.0
Health Insurance Costs [Abstract]
Percentage of our health insurance coverage provided by United	87.00%		87.00%
Number of days in advance of the beginning of a reporting quarter United establishes cash funding rates			90 days
Required accumulated cash surplus	$ 9.0		$ 9.0		9.0
Required deposit equal to approximately one day of claims funding activity	7.0		7.0
Prepaid health insurance, current	19.0		19.0
Amount which plan costs were less than the net premiums paid and owed	28.0		28.0
Premiums owed to United	(39.0)		(39.0)
Benefits costs incurred (reduced) related to run-off	(32.0)	(14.0)	(32.0)	(14.0)
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence							$ 1.5	$ 1.0
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year							$ 6.0	$ 6.0
Decrease Increase in accrued workers' compensation costs for changes in estimated losses			$ (25.0)	$ (26.0)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			4.30%	4.20%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					220.0	$ 229.0
Workers' Compensation Expense			$ 48.0	$ 44.0
Workers' Compensation Discount, Changed during period			(10.0)	(10.0)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			(51.0)	(41.0)
Workers' Compensation Liability	207.0	222.0	207.0	222.0
WorkersCompensationLiabilityNetOfAdminFeesCurrent	67.0	51.0	67.0	51.0
Workers' Compensation Liability, Noncurrent	140.0	171.0	140.0	171.0	163.0
Workers' Compensation Liability	207.0	222.0	207.0	222.0
Workers compensation administrative fees accrued	3.0	4.0	3.0	4.0
Undiscounted accrued workers' compensation costs			$ 242.0	250.0
Time period incurred claims expected to be paid recorded as restricted cash			1 year
Time period incurred claims expected to be paid included in deposits			Greater than 1 year
Restricted cash	67.0	51.0	$ 67.0	51.0	57.0	50.0
Deposits workers compensation	175.0	197.0	175.0	197.0	$ 198.0	$ 196.0
WorkersCompDepositsHeldInTrust	233.0		$ 233.0
Time period estimate of incurred claim costs to be paid included in short-term liabilities			1 year
Time period estimate of incurred claim costs to be paid included in long term liabilities			Greater than 1 year
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues	419.0	415.0	$ 1,365.0	1,333.0
Revenue Composition [Abstract]
Revenues	419.0	415.0	1,365.0	1,333.0
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues	223.0	222.0	694.0	682.0
Revenue Composition [Abstract]
Revenues	223.0	222.0	694.0	682.0
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues	286.0	282.0	902.0	886.0
Revenue Composition [Abstract]
Revenues	286.0	282.0	902.0	886.0
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues	294.0	295.0	938.0	943.0
Revenue Composition [Abstract]
Revenues	294.0	295.0	938.0	943.0
West [Member]
Disaggregation of Revenue [Line Items]
Revenues	321.0	321.0	1,016.0	1,014.0
Revenue Composition [Abstract]
Revenues	321.0	321.0	1,016.0	1,014.0
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	18.0	16.0	53.0	48.0
Revenue Composition [Abstract]
Revenues	$ 18.0	$ 16.0	$ 53.0	$ 48.0