Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Oct. 01, 2010
Health Insurance Costs [Abstract]
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	5,100		5,100
Amount which plan costs were less than the net premiums paid and owed	12,300		12,300
Prepaid health insurance, current	3,300		3,300
Premiums owed to United	19,000		19,000
Benefits costs incurred (reduced) related to run-off	(1,900)		(1,900)
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							$ 5,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses			$ (10,900)	$ (8,000)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)			2.50%	1.50%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,			$ 207,630	$ 183,928
Accrued claims			33,260	34,242
Present value discount			(3,360)	(2,008)
Paid claims			(21,686)	(20,044)
Ending balance	215,844	$ 196,118	215,844	196,118
Current portion of accrued claims	41,827	41,703	41,827	41,703
Long-term portion of accrued claims	174,017	154,415	174,017	154,415	166,493
Workers compensation administrative fees accrued	3,500		3,500
Undiscounted accrued workers' compensation costs			$ 230,400	207,200
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	41,827	41,703	$ 41,827	41,703	41,137	$ 42,637
Deposits workers compensation	165,150	157,850	$ 165,150	157,850	$ 154,215	$ 143,938
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
Disaggregation of Revenue [Line Items]
Revenue, Net	922,295	795,552	$ 1,936,667	1,678,216
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	239,278	204,285	511,642	440,385
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	108,264	92,049	223,452	190,855
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	153,025	131,099	321,092	273,876
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	213,920	185,094	446,785	389,201
West [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	194,986	170,452	407,605	357,896
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue, Net	$ 12,822	$ 12,573	$ 26,091	$ 26,003