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Accounting Policies (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Dec. 31, 2022
Oct. 01, 2019
Sep. 30, 2019
Revenue from Contract with Customer [Abstract]            
Unbilled $ 712.0   $ 669.0      
Disaggregation of Revenue [Line Items]            
Revenues(1) 1,802.0 $ 1,770.0        
Revenue Composition [Abstract]            
Gross billings 11,483.0 11,451.0        
Worksite employee payroll cost 9,681.0 9,681.0        
Revenue from Contract with Customer, Excluding Assessed Tax $ 1,802.0 1,770.0        
Health Insurance Costs [Abstract]            
Percentage of our health insurance coverage provided by United 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      
Required deposit equal to approximately one day of claims funding activity 7.0          
Prepaid health insurance, current 24.0          
Amount which plan costs were less than the net premiums paid and owed 33.0          
Premiums owed to United (63.0)          
Benefits costs incurred (reduced) related to run-off 18.0 9.0        
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 $ (9.0) $ (8.0)        
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 $ 14.0 $ 16.0        
Workers' Compensation Discount, Changed during period (3.0) (3.0)        
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid (16.0) (13.0)        
Workers' Compensation Liability 215.0 229.0        
WorkersCompensationLiabilityNetOfAdminFeesCurrent 60.0 48.0        
Workers' Compensation Liability, Noncurrent 155.0 181.0 163.0      
Workers' Compensation Liability 215.0 229.0        
Workers compensation administrative fees accrued 4.0          
Undiscounted accrued workers' compensation costs $ 247.0 253.0        
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 $ 61.0 48.0 57.0 50.0    
Deposits workers compensation $ 204.0 211.0 $ 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(1) $ 509.0 492.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax 509.0 492.0        
Southeast [Member]            
Disaggregation of Revenue [Line Items]            
Revenues(1) 246.0 239.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax 246.0 239.0        
Central [Member]            
Disaggregation of Revenue [Line Items]            
Revenues(1) 324.0 319.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax 324.0 319.0        
Southwest [Member]            
Disaggregation of Revenue [Line Items]            
Revenues(1) 340.0 340.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax 340.0 340.0        
West [Member]            
Disaggregation of Revenue [Line Items]            
Revenues(1) 366.0 363.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax 366.0 363.0        
Other Revenues [Member]            
Disaggregation of Revenue [Line Items]            
Revenues(1) 17.0 17.0        
Revenue Composition [Abstract]            
Revenue from Contract with Customer, Excluding Assessed Tax $ 17.0 $ 17.0