Accounting Policies (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 01, 2019	Dec. 31, 2018
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	$ 9,000,000.0		$ 9,000,000.0		$ 9,000,000
Required deposit equal to approximately one day of claims funding activity	6,500,000		6,500,000
Amount which plan costs were less than the net premiums paid and owed	32,800,000		32,800,000
Prepaid health insurance, current	23,800,000		23,800,000
Premiums owed to United	38,300,000		38,300,000
Benefits costs incurred (reduced) related to run-off	(1,900,000)	$ 2,900,000	(1,900,000)	$ 2,900,000
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence	1,000,000		1,000,000			$ 1,500,000
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year	6,000,000		6,000,000			$ 6,000,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses			(32,400,000)	(26,000,000.0)
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments					242,904,000		$ 229,639,000
Workers' Compensation Expense			33,397,000	44,334,000
Present value discount, net of accretion			(500,000)	(3,664,000)
Paid claims			(34,289,000)	(34,669,000)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	45,265,000	45,250,000	45,265,000	45,250,000
Long-term portion of accrued claims	196,247,000	190,390,000	196,247,000	190,390,000	193,609,000
Ending balance	241,512,000	235,640,000	241,512,000	235,640,000
Workers compensation administrative fees accrued	2,700,000		2,700,000
Undiscounted accrued workers' compensation costs			$ 259,400,000	255,300,000
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	45,265,000	45,251,000	$ 45,265,000	45,251,000	49,295,000		42,227,000
Deposits workers compensation	181,634,000	170,032,000	$ 181,634,000	170,032,000	175,913,000		$ 166,474,000
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
ReturnofExcessClaimFunds			$ 24,800,000	16,700,000
Revenue from Contract with Customer [Abstract]
Unbilled	520,900,000		520,900,000		$ 448,100,000
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,007,820,000	1,043,388,000	3,230,669,000	3,239,714,000
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	278,712,000	271,839,000	896,113,000	853,263,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	121,439,000	122,992,000	379,614,000	373,044,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	182,245,000	180,602,000	570,559,000	555,797,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	217,527,000	242,200,000	712,185,000	756,495,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	195,412,000	212,618,000	634,533,000	661,460,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 12,485,000	$ 13,137,000	$ 37,665,000	$ 39,655,000