v3.26.3
Accounting Policies (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Oct. 01, 2019
Sep. 30, 2019
Revenue from Contract with Customer [Abstract]                
Unbilled $ 900.0   $ 900.0   $ 810.0      
Disaggregation of Revenue [Line Items]                
Revenues 1,651.0 $ 1,623.0 5,232.0 $ 5,144.0        
Revenue Composition [Abstract]                
Gross billings 11,059.0 10,729.0 34,116.0 33,431.0        
Worksite employee payroll cost 9,408.0 9,106.0 28,884.0 28,287.0        
Revenues 1,651.0 1,623.0 $ 5,232.0 5,144.0        
Health Insurance Costs [Abstract]                
Percentage of our health insurance coverage provided by United     86.00%          
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 50.0   50.0          
Premiums owed to United (63.0)   (63.0)          
Benefits costs incurred (reduced) related to run-off     (4.0) 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     $ (7.0) $ (20.0)        
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)     4.00% 4.00%        
Incurred but not paid workers compensation liabilities [Abstract]                
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments         184.0 $ 204.0    
Workers' Compensation Expense     $ 71.0 $ 52.0        
Workers' Compensation Discount, Changed during period     (9.0) (9.0)        
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid     (63.0) (61.0)        
Workers' Compensation Liability 183.0 186.0 183.0 186.0        
WorkersCompensationLiabilityNetOfAdminFeesCurrent 84.0 79.0 84.0 79.0        
Workers' Compensation Liability, Noncurrent 99.0 107.0 99.0 107.0 102.0      
Workers' Compensation Liability 183.0 186.0 183.0 186.0        
Workers compensation administrative fees accrued 2.0 2.0 2.0 2.0        
Undiscounted accrued workers' compensation costs     $ 229.0 227.0        
Time period incurred claims expected to be paid recorded as restricted cash     1 year          
Restricted cash 84.0 79.0 $ 84.0 79.0 82.0 69.0    
Deposits workers compensation 132.0 147.0 132.0 147.0 $ 148.0 $ 178.0    
WorkersCompDepositsHeldInTrust 211.0   $ 211.0          
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 429.0 440.0 $ 1,402.0 1,418.0        
Revenue Composition [Abstract]                
Revenues 429.0 440.0 1,402.0 1,418.0        
Southeast [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 242.0 236.0 763.0 731.0        
Revenue Composition [Abstract]                
Revenues 242.0 236.0 763.0 731.0        
Central [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 302.0 295.0 952.0 931.0        
Revenue Composition [Abstract]                
Revenues 302.0 295.0 952.0 931.0        
Southwest [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 312.0 304.0 976.0 962.0        
Revenue Composition [Abstract]                
Revenues 312.0 304.0 976.0 962.0        
West [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 349.0 333.0 1,088.0 1,049.0        
Revenue Composition [Abstract]                
Revenues 349.0 333.0 1,088.0 1,049.0        
Other Revenues [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 17.0 15.0 51.0 53.0        
Revenue Composition [Abstract]                
Revenues $ 17.0 $ 15.0 $ 51.0 $ 53.0