Accounting Policies (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	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	22,600
Prepaid health insurance, current	13,600
Premiums owed to United	22,400
Benefits costs incurred related to run-off	(500)
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	$ 8,000	$ 2,600
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	1.50%	1.00%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,	$ 183,928	$ 162,184
Accrued claims	34,242	35,045
Present value discount	(2,008)	(1,274)
Paid claims	(20,044)	(19,038)
Ending balance	196,118	176,917
Current portion of accrued claims	41,703	41,236
Long-term portion of accrued claims	154,415	135,681	141,291
Workers compensation administrative fees accrued	2,200
Undiscounted accrued workers' compensation costs	$ 207,200	$ 187,000
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 - worker's compensation	$ 41,703		42,637
Deposits workers compensation	$ 157,850		$ 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