Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	600		600
Prepaid health insurance, current	(8,400)		(8,400)
Premiums owed to United	14,500		14,500
Benefits costs incurred (reduced) related to run-off	(5,700)	$ 3,000	(5,700)	$ 3,000
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			$ 23,000	$ 22,200
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			0.50%	0.80%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					240,761	$ 242,904
Workers' Compensation Expense			$ 20,559	$ 21,888
Present value discount, net of accretion			407	223
Paid claims			(22,904)	(22,298)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	46,120	45,216	46,120	45,216
Long-term portion of accrued claims	192,703	197,501	192,703	197,501	195,239
Ending balance	238,823	242,717	238,823	242,717
Workers compensation administrative fees accrued	3,700		3,700
Undiscounted accrued workers' compensation costs			$ 254,800	261,400
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	46,128	45,216	$ 46,128	45,216	45,522	49,295
Deposits workers compensation	203,612	177,944	$ 203,612	177,944	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	585,400		$ 585,400		$ 380,800
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,185,371	993,366	2,472,206	2,222,849
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	329,016	273,543	702,645	617,401
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	150,810	117,013	307,070	258,175
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	206,084	177,022	431,784	388,314
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	238,368	222,734	495,348	494,658
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	247,184	192,310	508,538	439,121
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 13,909	$ 10,744	$ 26,821	$ 25,180