Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
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	6,000
Amount which plan costs were less than the net premiums paid and owed	19,500
Prepaid health insurance, current	10,500
Premiums owed to United	40,600
Benefits costs incurred (reduced) related to run-off		$ 3,800
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence	1,000
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year	6,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses	$ 7,800	$ 5,700
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	2.50%	2.50%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,	$ 229,639	$ 207,630
Accrued claims	15,787	16,903
Present value discount	(1,689)	(1,692)
Paid claims	(12,409)	(9,931)
Ending balance	231,328	212,910
Current portion of accrued claims	44,704	41,137
Long-term portion of accrued claims	186,624	171,773	187,412
Workers compensation administrative fees accrued	3,300
Undiscounted accrued workers' compensation costs	$ 250,200	226,400
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	$ 44,705	41,137	42,227	$ 41,137
Deposits workers compensation	$ 170,905	160,840	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
Unbilled	$ 411,742		$ 385,567
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,153,010	1,014,372
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	310,945	272,363
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	129,906	115,188
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	195,753	168,068
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	269,832	232,866
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	233,204	212,619
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 13,370	$ 13,268