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Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Oct. 01, 2019	Sep. 30, 2019
Revenue from Contract with Customer [Abstract]
Unbilled	$ 727.0		$ 727.0		$ 669.0
Disaggregation of Revenue [Line Items]
Revenues	1,605.0	$ 1,585.0	3,407.0	$ 3,355.0
Revenue Composition [Abstract]
Gross billings	10,361.0	10,245.0	21,844.0	21,696.0
Worksite employee payroll cost	8,756.0	8,660.0	18,437.0	18,341.0
Revenues	$ 1,605.0	1,585.0	$ 3,407.0	3,355.0
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	18.0		18.0
Amount which plan costs were less than the net premiums paid and owed	27.0		27.0
Premiums owed to United	(35.0)		(35.0)
Benefits costs incurred (reduced) related to run-off	(26.0)	(10.0)	(26.0)	(10.0)
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			$ (17.0)	$ (15.0)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			4.50%	4.00%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					220.0	$ 229.0
Workers' Compensation Expense			$ 31.0	$ 31.0
Workers' Compensation Discount, Changed during period			(7.0)	(6.0)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			(33.0)	(26.0)
Workers' Compensation Liability	211.0	228.0	211.0	228.0
WorkersCompensationLiabilityNetOfAdminFeesCurrent	65.0	48.0	65.0	48.0
Workers' Compensation Liability, Noncurrent	146.0	180.0	146.0	180.0	163.0
Workers' Compensation Liability	211.0	228.0	211.0	228.0
Workers compensation administrative fees accrued	2.0	3.0	2.0	3.0
Undiscounted accrued workers' compensation costs			$ 244.0	253.0
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	64.0	48.0	$ 64.0	48.0	57.0	50.0
Deposits workers compensation	170.0	181.0	$ 170.0	181.0	$ 198.0	$ 196.0
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	437.0	426.0	$ 946.0	918.0
Revenue Composition [Abstract]
Revenues	437.0	426.0	946.0	918.0
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues	225.0	221.0	471.0	460.0
Revenue Composition [Abstract]
Revenues	225.0	221.0	471.0	460.0
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues	292.0	285.0	616.0	604.0
Revenue Composition [Abstract]
Revenues	292.0	285.0	616.0	604.0
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues	304.0	308.0	644.0	648.0
Revenue Composition [Abstract]
Revenues	304.0	308.0	644.0	648.0
West [Member]
Disaggregation of Revenue [Line Items]
Revenues	329.0	330.0	695.0	693.0
Revenue Composition [Abstract]
Revenues	329.0	330.0	695.0	693.0
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues	18.0	15.0	35.0	32.0
Revenue Composition [Abstract]
Revenues	$ 18.0	$ 15.0	$ 35.0	$ 32.0

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The maximum amount the Company will bear of the economic burden per policy year for those claims exceeding the first layer maximum per claim occurrence.

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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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-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

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