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Accounting Policies (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Dec. 31, 2023
Oct. 01, 2019
Sep. 30, 2019
Revenue from Contract with Customer [Abstract]            
Unbilled $ 819.0   $ 810.0      
Disaggregation of Revenue [Line Items]            
Revenues 1,863.0 $ 1,802.0        
Revenue Composition [Abstract]            
Gross billings 12,144.0 11,483.0        
Worksite employee payroll cost 10,281.0 9,681.0        
Revenues $ 1,863.0 1,802.0        
Health Insurance Costs [Abstract]            
Percentage of our health insurance coverage provided by United 83.00%          
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      
Required deposit equal to approximately one day of claims funding activity 7.0          
Prepaid health insurance, current 43.0          
Amount which plan costs were less than the net premiums paid and owed 52.0          
Premiums owed to United (64.0)          
Benefits costs incurred (reduced) related to run-off 12.0 18.0        
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 $ (7.0) $ (9.0)        
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths) 4.00% 4.50%        
Incurred but not paid workers compensation liabilities [Abstract]            
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments     204.0 $ 220.0    
Workers' Compensation Expense $ 16.0 $ 14.0        
Workers' Compensation Discount, Changed during period (3.0) (3.0)        
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid (21.0) (16.0)        
Workers' Compensation Liability 196.0 215.0        
WorkersCompensationLiabilityNetOfAdminFeesCurrent 74.0 60.0        
Workers' Compensation Liability, Noncurrent 122.0 155.0 135.0      
Workers' Compensation Liability 196.0 215.0        
Workers compensation administrative fees accrued 2.0 4.0        
Undiscounted accrued workers' compensation costs $ 234.0 247.0        
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 $ 74.0 61.0 69.0 57.0    
Deposits workers compensation 171.0 204.0 $ 178.0 $ 198.0    
WorkersCompDepositsHeldInTrust $ 242.0          
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 $ 526.0 509.0        
Revenue Composition [Abstract]            
Revenues 526.0 509.0        
Southeast [Member]            
Disaggregation of Revenue [Line Items]            
Revenues 257.0 246.0        
Revenue Composition [Abstract]            
Revenues 257.0 246.0        
Central [Member]            
Disaggregation of Revenue [Line Items]            
Revenues 335.0 324.0        
Revenue Composition [Abstract]            
Revenues 335.0 324.0        
Southwest [Member]            
Disaggregation of Revenue [Line Items]            
Revenues 347.0 340.0        
Revenue Composition [Abstract]            
Revenues 347.0 340.0        
West [Member]            
Disaggregation of Revenue [Line Items]            
Revenues 377.0 366.0        
Revenue Composition [Abstract]            
Revenues 377.0 366.0        
Other Revenues [Member]            
Disaggregation of Revenue [Line Items]            
Revenues 21.0 17.0        
Revenue Composition [Abstract]            
Revenues $ 21.0 $ 17.0