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Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Oct. 01, 2019	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Unbilled	$ 882.0		$ 882.0		$ 810.0
Disaggregation of Revenue [Line Items]
Revenues	1,623.0	$ 1,561.0	5,144.0	$ 4,968.0
Revenue Composition [Abstract]
Gross billings	10,729.0	10,291.0	33,431.0	32,135.0
Worksite employee payroll cost	9,106.0	8,730.0	28,287.0	27,167.0
Revenues	$ 1,623.0	1,561.0	$ 5,144.0	4,968.0
Health Insurance Costs [Abstract]
Percentage of our health insurance coverage provided by United	84.00%		84.00%
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	10.0		10.0
Amount which plan costs were less than the net premiums paid and owed	19.0		19.0
Premiums owed to United	(26.0)		(26.0)
Benefits costs incurred (reduced) related to run-off	9.0	(32.0)	9.0	(32.0)
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			$ (20.0)	$ (25.0)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			4.00%	4.30%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					204.0	$ 220.0
Workers' Compensation Expense			$ 52.0	$ 48.0
Workers' Compensation Discount, Changed during period			(9.0)	(10.0)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			(61.0)	(51.0)
Workers' Compensation Liability	186.0	207.0	186.0	207.0
WorkersCompensationLiabilityNetOfAdminFeesCurrent	79.0	67.0	79.0	67.0
Workers' Compensation Liability, Noncurrent	107.0	140.0	107.0	140.0	135.0
Workers' Compensation Liability	186.0	207.0	186.0	207.0
Workers compensation administrative fees accrued	2.0	3.0	2.0	3.0
Undiscounted accrued workers' compensation costs			$ 227.0	242.0
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	79.0	67.0	$ 79.0	67.0	69.0	57.0
Deposits workers compensation	147.0	175.0	147.0	175.0	$ 178.0	$ 198.0
WorkersCompDepositsHeldInTrust	226.0		$ 226.0
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			beyond one year
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues	440.0	419.0	$ 1,418.0	1,365.0
Revenue Composition [Abstract]
Revenues	440.0	419.0	1,418.0	1,365.0
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues	236.0	223.0	731.0	694.0
Revenue Composition [Abstract]
Revenues	236.0	223.0	731.0	694.0
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues	295.0	286.0	931.0	902.0
Revenue Composition [Abstract]
Revenues	295.0	286.0	931.0	902.0
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues	304.0	294.0	962.0	938.0
Revenue Composition [Abstract]
Revenues	304.0	294.0	962.0	938.0
West [Member]
Disaggregation of Revenue [Line Items]
Revenues	333.0	321.0	1,049.0	1,016.0
Revenue Composition [Abstract]
Revenues	333.0	321.0	1,049.0	1,016.0
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	15.0	18.0	53.0	53.0
Revenue Composition [Abstract]
Revenues	$ 15.0	$ 18.0	$ 53.0	$ 53.0

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Payroll costs incurred (including share-based compensation) by the company's worksite employees that are directly related to services rendered by the worksite employees during the reporting period.

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