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Accounting Policies (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Oct. 01, 2019 | Sep. 30, 2019
Revenue from Contract with Customer [Abstract] |  |  |  |  |  |  |  |
Unbilled | $ 727.0 |  | $ 727.0 |  | $ 669.0 |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 1,605.0 | $ 1,585.0 | 3,407.0 | $ 3,355.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Gross billings | 10,361.0 | 10,245.0 | 21,844.0 | 21,696.0 |  |  |  |
Worksite employee payroll cost | 8,756.0 | 8,660.0 | 18,437.0 | 18,341.0 |  |  |  |
Revenues | $ 1,605.0 | 1,585.0 | $ 3,407.0 | 3,355.0 |  |  |  |
Health Insurance Costs [Abstract] |  |  |  |  |  |  |  |
Percentage of our health insurance coverage provided by United | 87.00% |  | 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 |  | 9.0 |  |  |
Required deposit equal to approximately one day of claims funding activity | 7.0 |  | 7.0 |  |  |  |  |
Prepaid health insurance, current | 18.0 |  | 18.0 |  |  |  |  |
Amount which plan costs were less than the net premiums paid and owed | 27.0 |  | 27.0 |  |  |  |  |
Premiums owed to United | (35.0) |  | (35.0) |  |  |  |  |
Benefits costs incurred (reduced) related to run-off | (26.0) | (10.0) | (26.0) | (10.0) |  |  |  |
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 |  |  | $ (17.0) | $ (15.0) |  |  |  |
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 |  |  | $ 31.0 | $ 31.0 |  |  |  |
Workers' Compensation Discount, Changed during period |  |  | (7.0) | (6.0) |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid |  |  | (33.0) | (26.0) |  |  |  |
Workers' Compensation Liability | 211.0 | 228.0 | 211.0 | 228.0 |  |  |  |
WorkersCompensationLiabilityNetOfAdminFeesCurrent | 65.0 | 48.0 | 65.0 | 48.0 |  |  |  |
Workers' Compensation Liability, Noncurrent | 146.0 | 180.0 | 146.0 | 180.0 | 163.0 |  |  |
Workers' Compensation Liability | 211.0 | 228.0 | 211.0 | 228.0 |  |  |  |
Workers compensation administrative fees accrued | 2.0 | 3.0 | 2.0 | 3.0 |  |  |  |
Undiscounted accrued workers' compensation costs |  |  | $ 244.0 | 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 | 64.0 | 48.0 | $ 64.0 | 48.0 | 57.0 | 50.0 |  |
Deposits workers compensation | 170.0 | 181.0 | $ 170.0 | 181.0 | $ 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 | 437.0 | 426.0 | $ 946.0 | 918.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | 437.0 | 426.0 | 946.0 | 918.0 |  |  |  |
Southeast [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 225.0 | 221.0 | 471.0 | 460.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | 225.0 | 221.0 | 471.0 | 460.0 |  |  |  |
Central [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 292.0 | 285.0 | 616.0 | 604.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | 292.0 | 285.0 | 616.0 | 604.0 |  |  |  |
Southwest [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 304.0 | 308.0 | 644.0 | 648.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | 304.0 | 308.0 | 644.0 | 648.0 |  |  |  |
West [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 329.0 | 330.0 | 695.0 | 693.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | 329.0 | 330.0 | 695.0 | 693.0 |  |  |  |
Other Revenues [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenues | 18.0 | 15.0 | 35.0 | 32.0 |  |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |  |  |
Revenues | $ 18.0 | $ 15.0 | $ 35.0 | $ 32.0 |  |  |  |

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- Definition Carrying value as of the balance sheet date of obligations and payables pertaining to claims incurred of a workers compensation nature. Used to reflect the noncurrent portion of the liabilities (due beyond one year; or beyond one operating cycle if longer). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 + Details
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