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Accounting Policies (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Oct. 01, 2019 | Sep. 30, 2019
Revenue from Contract with Customer [Abstract] |  |  |  |  |  |
Unbilled | $ 668,920 | $ 600,446 |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 6,485,871 | 5,938,818 | $ 4,973,070 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Gross billings | 43,141,366 | 40,126,910 | 33,318,693 |  |  |
Worksite employee payroll cost | 36,655,495 | 34,188,092 | 28,345,623 |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 6,485,871 | 5,938,818 | 4,973,070 |  |  |
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 | $ 9,000 |  |  |  |  |
Required deposit equal to approximately one day of claims funding activity | 6,500 |  |  |  |  |
Prepaid health insurance, current | (23,500) |  |  |  |  |
Amount which plan costs were less than the net premiums paid and owed | (32,500) |  |  |  |  |
Premiums owed to United | (6,500) |  |  |  |  |
Benefits costs incurred (reduced) related to run-off | 13,000 | (12,100) |  |  |  |
Workers' Compensation Costs [Abstract] |  |  |  |  |  |
Company's maximum economic burden for the first layer of claims per occurrence |  |  |  |  | $ 1,500 | $ 1,000
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year |  |  |  |  | $ 6,000 | $ 6,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses | $ (33,500) | $ (42,200) | (41,700) |  |  |
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths) | 4.30% | 2.90% |  |  |  |
Incurred but not paid workers compensation liabilities [Abstract] |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments |  | $ 229,408 | 239,623 |  |  |
Workers' Compensation Expense | $ 61,720 | 49,121 |  |  |  |
Workers' Compensation Discount, Changed during period | (13,430) | (9,517) |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid | (57,443) | (49,819) |  |  |  |
Workers' Compensation Liability | 220,255 | 229,408 |  |  |  |
WorkersCompensationLiabilityNetOfAdminFeesCurrent | 57,403 | 49,779 |  |  |  |
Workers' Compensation Liability, Noncurrent | 162,852 | 179,629 |  |  |  |
Workers' Compensation Liability | 220,255 | 229,408 |  |  |  |
Workers compensation administrative fees accrued | 3,100 | 3,700 |  |  |  |
Undiscounted accrued workers' compensation costs | $ 250,000 | 250,500 |  |  |  |
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 |  |  |  |  |
ReturnOfExcessClaimFunds | $ 46,300 |  |  |  |  |
Restricted cash | 57,403 | 49,779 | 46,929 | $ 45,522 |  |
Deposits workers compensation | $ 198,170 | 196,370 | 185,027 | $ 186,331 |  |
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) | $ 1,756,884 | 1,624,556 | 1,390,156 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 1,756,884 | 1,624,556 | 1,390,156 |  |  |
Southeast [Member] |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 906,712 | 796,219 | 630,342 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 906,712 | 796,219 | 630,342 |  |  |
Central [Member] |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 1,170,491 | 1,045,043 | 867,914 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 1,170,491 | 1,045,043 | 867,914 |  |  |
Southwest [Member] |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 1,249,922 | 1,163,088 | 993,747 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 1,249,922 | 1,163,088 | 993,747 |  |  |
West [Member] |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 1,337,294 | 1,251,186 | 1,033,996 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 1,337,294 | 1,251,186 | 1,033,996 |  |  |
Other Revenues [Member] |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |
Revenues(1) | 64,568 | 58,726 | 56,915 |  |  |
Revenue Composition [Abstract] |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 64,568 | $ 58,726 | $ 56,915 |  |  |

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