Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	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		4,500
Amount which plan costs were less than the net premiums paid and owed		15,700
Prepaid health insurance, current		6,700
Premiums owed to United		16,900
Benefits costs incurred related to run-off		4,600
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		$ 9,100	$ 500
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)		1.00%	1.00%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,		$ 162,174	$ 136,088
Accrued claims		46,967	49,607
Present value discount		(1,785)	(1,816)
Paid claims		(29,537)	(28,735)
Ending balance		177,819	155,144
Current portion of accrued claims		41,778	43,056
Long-term portion of accrued claims		136,041	112,088	124,746
Workers compensation administrative fees accrued		2,200
Undiscounted accrued workers' compensation costs		$ 187,700	164,700
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
Return of Excess Claim Funds Net of Additional Amounts Paid in Claim Funds for Prior Policy Years		$ 12,800	$ 5,300
Restricted cash - worker's compensation		41,779		37,418
Deposits workers compensation		$ 134,588		$ 136,462
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
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share	$ 0.05