Accounting Policies (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	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
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	30,200,000
Prepaid health insurance, current	21,200,000
Premiums owed to United	19,200,000
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	10,400,000	8,600,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	0.70%	1.20%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,	104,791,000	96,934,000
Accrued claims	28,586,000	26,668,000
Present value discount	(712,000)	(1,159,000)
Paid claims	(23,133,000)	(21,123,000)
Ending balance	109,532,000	101,320,000
Current portion of accrued claims	46,069,000	42,812,000
Long-term portion of accrued claims	63,463,000	58,508,000	60,054,000
Ending balance	109,532,000	101,320,000
Return of excess claims funds related to ACE program	2,500,000	10,000,000
Workers compensation administrative fees accrued	2,300,000
Undiscounted accrued workers' compensation costs	121,700,000	115,100,000
Restricted cash - workers' compensation	46,069,000		44,737,000
Deposits - workers' compensation	$ 57,588,000		$ 52,320,000
Time period incurred claims expected to be paid recorded as restricted cash	1 year	1 year
Time period incurred claims expected to be paid, included in deposits, a long-term assets	Greater than 1Y	Greater than 1Y
Time period estimate of incurred claim costs to be paid included in short-term liabilities	1 year
Time period estimate of incurred claim costs to be paid included in long term liabilities	Greater than 1Y