v3.26.1
Accounting Policies (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Oct. 01, 2019
Sep. 30, 2019
Revenue from Contract with Customer [Abstract]                
Unbilled $ 867.0   $ 867.0   $ 810.0      
Disaggregation of Revenue [Line Items]                
Revenues 1,686.0 $ 1,658.0 3,581.0 $ 3,521.0        
Revenue Composition [Abstract]                
Gross billings 10,911.0 10,558.0 23,057.0 22,702.0        
Worksite employee payroll cost 9,225.0 8,900.0 19,476.0 19,181.0        
Revenues $ 1,686.0 1,658.0 $ 3,581.0 3,521.0        
Health Insurance Costs [Abstract]                
Percentage of our health insurance coverage provided by United 85.00%   85.00%          
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 40.0   40.0          
Premiums owed to United (55.0)   (55.0)          
Benefits costs incurred (reduced) related to run-off (4.0) 11.0 (4.0) 11.0        
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) $ (14.0)        
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)     3.80% 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     $ 44.0 $ 32.0        
Workers' Compensation Discount, Changed during period     (5.0) (6.0)        
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid     (39.0) (41.0)        
Workers' Compensation Liability 184.0 189.0 184.0 189.0        
WorkersCompensationLiabilityNetOfAdminFeesCurrent 81.0 76.0 81.0 76.0        
Workers' Compensation Liability, Noncurrent 103.0 113.0 103.0 113.0 102.0      
Workers' Compensation Liability 184.0 189.0 184.0 189.0        
Workers compensation administrative fees accrued 2.0 3.0 2.0 3.0        
Undiscounted accrued workers' compensation costs     $ 228.0 228.0        
Time period incurred claims expected to be paid recorded as restricted cash     1 year          
Restricted cash 81.0 76.0 $ 81.0 76.0 82.0 69.0    
Deposits workers compensation 160.0 174.0 160.0 174.0 $ 148.0 $ 178.0    
WorkersCompDepositsHeldInTrust 236.0   $ 236.0          
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 446.0 452.0 $ 973.0 978.0        
Revenue Composition [Abstract]                
Revenues 446.0 452.0 973.0 978.0        
Southeast [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 250.0 238.0 521.0 495.0        
Revenue Composition [Abstract]                
Revenues 250.0 238.0 521.0 495.0        
Central [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 308.0 301.0 650.0 636.0        
Revenue Composition [Abstract]                
Revenues 308.0 301.0 650.0 636.0        
Southwest [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 314.0 311.0 664.0 658.0        
Revenue Composition [Abstract]                
Revenues 314.0 311.0 664.0 658.0        
West [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 353.0 339.0 739.0 716.0        
Revenue Composition [Abstract]                
Revenues 353.0 339.0 739.0 716.0        
Other Revenues [Member]                
Disaggregation of Revenue [Line Items]                
Revenues 15.0 17.0 34.0 38.0        
Revenue Composition [Abstract]                
Revenues $ 15.0 $ 17.0 $ 34.0 $ 38.0