Accounting Policies (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	19,500,000		19,500,000
Prepaid health insurance, current	10,500,000		10,500,000
Premiums owed to United	23,400,000		23,400,000
Benefits costs incurred (reduced) related to run-off	3,000,000.0	$ (3,500,000)	3,000,000.0	$ (3,500,000)
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			$ 22,200,000	$ 19,000,000.0
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			0.80%	2.30%
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			$ 21,888,000	$ 27,299,000
Present value discount, net of accretion			223,000	(2,703,000)
Paid claims			(22,298,000)	(22,727,000)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	45,216,000	43,267,000	45,216,000	43,267,000
Long-term portion of accrued claims	197,501,000	188,241,000	197,501,000	188,241,000	193,609,000
Ending balance	242,717,000	231,508,000	242,717,000	231,508,000
Workers compensation administrative fees accrued	3,400,000		3,400,000
Undiscounted accrued workers' compensation costs			$ 261,400,000	250,900,000
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	45,216,000	43,268,000	$ 45,216,000	43,268,000	49,295,000		42,227,000
Deposits workers compensation	177,944,000	187,728,000	$ 177,944,000	187,728,000	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
Revenue from Contract with Customer [Abstract]
Unbilled	463,100,000		463,100,000		$ 448,100,000
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	993,366,000	1,043,316,000	2,222,849,000	2,196,326,000
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	273,543,000	270,475,000	617,401,000	581,422,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	117,013,000	120,148,000	258,175,000	250,054,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	177,022,000	179,443,000	388,314,000	375,196,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	222,734,000	244,462,000	494,658,000	514,294,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	192,310,000	215,639,000	439,121,000	448,842,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 10,744,000	$ 13,149,000	$ 25,180,000	$ 26,518,000