Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Oct. 01, 2010
Accounting Policies [Abstract]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,000
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		22,900
Accrued health insurance, current		13,900
Premiums owed to United		23,000
Benefits costs incurred related to run-off		$ 3,700
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
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,	$ 162,184	$ 162,184	$ 136,088
Accrued claims		35,045	32,720
Present value discount		(1,274)	(1,189)
Paid claims		(19,038)	(19,794)
Ending balance		176,917	147,825
Current portion of accrued claims		41,236	48,887
Long-term portion of accrued claims		135,681	98,938	124,746
Workers compensation administrative fees accrued		2,100
Undiscounted accrued workers' compensation costs		$ 187,000	$ 157,400
Time period incurred claims expected to be paid recorded as restricted cash		1 year
Return of Excess Claim Funds Net of Additional Amounts Paid in Claim Funds for Prior Policy Years		$ 12,800
Time period incurred claims expected to be paid, included in deposits, a long-term asset		Greater than 1 year
Restricted cash - worker's compensation		$ 41,226		37,418
Deposits workers compensation		$ 130,731		$ 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 Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share	$ 0.05