Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Required deposit equal to approximately one day of claims funding activity	5,100
Amount which plan costs were less than the net premiums paid and owed	32,000
Prepaid health insurance, current	23,000
Premiums owed to United	42,100
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					$ 5,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses	$ 5,700	$ 3,900
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	16,903	16,772
Present value discount	(1,692)	(968)
Paid claims	(9,931)	(10,480)
Ending balance	212,910	189,252
Current portion of accrued claims	41,137	42,458
Long-term portion of accrued claims	171,773	146,794	166,493
Workers compensation administrative fees accrued	2,700
Undiscounted accrued workers' compensation costs	$ 226,400	199,900
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 - worker's compensation	$ 41,137	$ 42,500	41,137	$ 42,600
Deposits workers compensation	$ 160,840		$ 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