Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Oct. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
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	5,000		5,000
Amount which plan costs were less than the net premiums paid and owed	17,400		17,400
Prepaid health insurance, current	8,400		8,400
Premiums owed to United	27,600		27,600
Benefits costs incurred (reduced) related to run-off	(2,900)	$ 1,300	(2,900)	$ 1,300
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			$ (26,000)	$ (13,400)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			2.00%	2.50%
Incurred but not paid workers compensation liabilities [Abstract]
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments						229,639	$ 207,630
Workers' Compensation Expense			$ 44,334	$ 54,744
Present value discount			(3,664)	(5,372)
Paid claims			(34,669)	(31,645)
WorkersCompensationLiabilityNetOfAdminFeesCurrent	45,250	42,258	45,250	42,258
Long-term portion of accrued claims	190,390	183,099	190,390	183,099		187,412
Ending balance	235,640	225,357	235,640	225,357
Workers compensation administrative fees accrued	3,700		3,700
Undiscounted accrued workers' compensation costs			$ 255,300	241,300
Time period incurred claims expected to be paid recorded as restricted cash			1 year
Return Of Excess Claim Funds			$ 16,700	19,400
Time period incurred claims expected to be paid, included in deposits, a long-term asset			Greater than 1 year
Restricted cash	45,251	42,257	$ 45,251	42,257		42,227	41,137
Deposits workers compensation	170,032	152,268	$ 170,032	152,268		166,474	$ 154,215
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	474,200		$ 474,200			$ 385,600
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,043,388	925,126	3,239,714	2,861,793
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	271,839	237,076	853,263	748,717
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	122,992	109,552	373,044	333,004
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	180,602	155,184	555,797	476,277
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	242,200	217,946	756,495	664,731
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	212,618	192,011	661,460	599,617
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 13,137	$ 13,357	$ 39,655	$ 39,447