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Workers' Compensation - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Workers Compensation Reserve [Abstract]        
Workers' compensation claims liabilities, Beginning balance $ 84,543 $ 55,765 $ 70,564 $ 51,193
Add: claims expense accrual:        
Current period 17,396 9,891 44,825 26,224
Prior periods 5,002 3,835 12,688 6,945
Total expense accrual 22,398 13,726 57,513 33,169
Less: claim payments related to:        
Current period 3,124 2,207 5,998 3,790
Prior periods 7,837 5,656 26,099 18,944
Total paid 10,961 7,863 32,097 22,734
Workers' compensation claims liabilities, Ending balance 95,980 61,628 95,980 61,628
Incurred but not reported (IBNR) $ 59,412 $ 44,311 $ 59,412 $ 44,311