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