Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Oct. 01, 2019	Sep. 30, 2019
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	$ 9,000		$ 9,000		$ 9,000
Required deposit equal to approximately one day of claims funding activity	6,500		6,500
Prepaid health insurance, current	18,800		18,800
Amount which plan costs were less than the net premiums paid and owed	9,800		9,800
Premiums owed to United	(42,900)		(42,900)
Benefits costs incurred (reduced) related to run-off	13,900	$ (12,800)	13,900	$ (12,800)
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			$ (26,200)	$ (35,300)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			4.20%	2.50%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					229,408	$ 239,623
Workers' Compensation Expense			$ 44,292	$ 33,467
Workers' Compensation Discount, Changed during period			(10,233)	(6,485)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			(41,595)	(40,112)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	51,263	51,829	51,263	51,829
Workers' Compensation Liability, Noncurrent	170,609	174,664	170,609	174,664	179,629
Workers' Compensation Liability	221,872	226,493	221,872	226,493
Workers compensation administrative fees accrued	3,700		3,700
Undiscounted accrued workers' compensation costs			$ 249,500	245,500
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			$ 43,400
Restricted cash	51,263		51,263		49,779
Deposits workers compensation	197,466	179,014	$ 197,466	179,014	196,370	$ 185,027
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
Revenue from Contract with Customer [Abstract]
Unbilled	644,400		$ 644,400		$ 600,400
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,550,887	1,439,160	4,905,668	4,449,104
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	415,318	385,541	1,333,456	1,224,142
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	221,930	197,131	681,866	592,116
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	282,248	256,150	885,960	779,500
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	295,142	280,299	942,929	869,073
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	320,567	305,465	1,013,669	940,617
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 15,682	$ 14,574	$ 47,788	$ 43,656