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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 |  |  |  |

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