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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 |  |  |  |

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