Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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%
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
Required deposit equal to approximately one day of claims funding activity	6,500
Amount which plan costs were less than the net premiums paid and owed	51,400
Prepaid health insurance, current	42,400
Premiums owed to United	66,000
Benefits costs incurred (reduced) related to run-off	5,500	$ (1,500)
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence	1,000				$ 1,500
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	$ 13,200	$ 10,100
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	0.50%	1.00%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments			240,761	$ 242,904
Workers' Compensation Expense	$ 8,522	$ 14,339
Present value discount, net of accretion	652	(200)
Paid claims	(13,246)	(11,121)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	46,353	48,289
Long-term portion of accrued claims	190,336	197,633	195,239
Ending balance	236,689	245,922
Workers compensation administrative fees accrued	3,300
Undiscounted accrued workers' compensation costs	$ 253,200	265,600
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	$ 46,353	48,349	45,522	49,295
Deposits workers compensation	$ 193,067	188,549	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	$ 549,000		$ 380,800
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,286,835	1,229,483
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	373,629	344,086
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	156,260	140,678
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	225,700	211,302
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	256,979	272,130
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	261,354	246,853
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 12,913	$ 14,434