Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Oct. 01, 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
Amount which plan costs were less than the net premiums paid and owed	15,300		15,300
Prepaid health insurance, current	6,300		6,300
Premiums owed to United	33,700		33,700
Benefits costs incurred (reduced) related to run-off	(4,500)	$ 1,900	(4,500)	$ 1,900
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence	1,000		1,000				$ 1,500
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year	6,000		6,000				$ 6,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses			$ (31,400)	$ (32,400)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			0.50%	0.70%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					240,761	$ 242,904
Workers' Compensation Expense			$ 34,349	$ 33,397
Present value discount, net of accretion			476	(500)
Paid claims			(36,580)	(34,289)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	47,813	45,265	47,813	45,265
Long-term portion of accrued claims	191,193	196,247	191,193	196,247	195,239
Ending balance	239,006	241,512	239,006	241,512
Workers compensation administrative fees accrued	3,600		3,600
Undiscounted accrued workers' compensation costs			$ 254,500	259,400
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, a long-term asset			Greater than 1 year
Restricted cash	47,813	45,265	$ 47,813	45,265	45,522	49,295
Deposits workers compensation	208,236	181,634	$ 208,236	181,634	186,331	$ 175,913
Time period estimate of incurred claim costs to be paid included in short-term liabilities			1 year
Time period incurred claims expected to be paid, included in long-term liabilities			Greater than 1 year
Revenue from Contract with Customer [Abstract]
Unbilled	564,600		$ 564,600		$ 380,800
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,209,628	1,007,820	3,681,834	3,230,669
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	332,587	278,712	1,035,233	896,113
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	155,555	121,439	462,625	379,614
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	211,315	182,245	643,100	570,559
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	241,473	217,527	736,820	712,185
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	253,562	195,412	762,100	634,533
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 15,136	$ 12,485	$ 41,956	$ 37,665