Accounting Policies (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	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	2,800,000
Amount which plan costs were less than the net premiums paid and owed	14,900,000
Prepaid health insurance, current	5,900,000
Premiums owed to United	1,900,000
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	6,500,000	6,700,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	0.50%	0.80%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,	111,685,000	104,791,000
Accrued claims	19,194,000	19,164,000
Present value discount	(345,000)	(532,000)
Paid claims	(16,199,000)	(15,179,000)
Ending balance	114,335,000	108,244,000
Current portion of accrued claims	47,467,000	44,580,000
Long-term portion of accrued claims	66,868,000	63,664,000	64,536,000
Ending balance	114,335,000	108,244,000
Workers compensation administrative fees accrued	2,800,000
Undiscounted accrued workers' compensation costs	125,100,000	120,900,000
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	Greater than 1 year
Return of excess claim funds	0	2,500,000
Restricted cash - worker's compensation	47,467,000		47,149,000
Deposits workers compensation	$ 69,947,000		$ 64,201,000
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