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Workers' Compensation (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Summarizes aggregate workers' compensation reserve activity        
Workers' compensation claims liabilities, Beginning balance $ 55,765 $ 40,996 $ 51,200 $ 39,301
Add: claims expense accrual:        
Current period 9,891 6,187 26,224 16,172
Prior periods 3,835 946 6,945 1,468
Total expense accrual 13,726 7,133 33,169 17,640
Less: claim payments related to:        
Current period 2,207 1,736 3,790 2,955
Prior periods 5,656 3,058 18,944 10,651
Total paid 7,863 4,794 22,734 13,606
Workers' compensation claims liabilities, Ending balance 61,600 43,335 61,600 43,335
Incurred but not reported (IBNR) $ 44,311 $ 31,178 $ 44,311 $ 31,178