Accounting Policies (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Oct. 01, 2010	Sep. 30, 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	$ 9,000,000		$ 9,000,000
Required deposit equal to approximately one day of claims funding activity	3,500,000
Amount which plan costs were less than the net premiums paid and owed	34,100,000
Prepaid health insurance, current	25,100,000
Premiums owed to United	22,300,000
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence				1,000,000	1,000,000
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year				5,000,000
Reduction in accrued workers' compensation costs for changes in estimated losses	700,000	3,600,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	1.00%	0.50%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,	120,833,000	111,685,000
Accrued claims	12,453,000	9,134,000
Present value discount	(439,000)	(160,000)
Paid claims	(9,399,000)	(7,583,000)
Ending balance	123,448,000	113,076,000
Current portion of accrued claims	52,163,000	47,374,000
Long-term portion of accrued claims	71,285,000	65,702,000	68,905,000
Ending balance	123,448,000	113,076,000
Workers compensation administrative fees accrued	1,200,000
Undiscounted accrued workers' compensation costs	133,600,000	124,300,000
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	52,162,000		51,928,000
Deposits workers compensation	$ 84,871,000		$ 81,878,000
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