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Workers' Compensation - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Workers Compensation Reserve [Abstract]        
Workers' compensation claims liabilities, Beginning balance $ 222,265 $ 120,135 $ 215,982 $ 112,444
Add: claims expense accrual:        
Current period 28,983 17,548 55,219 33,899
Prior periods (4,387) 1,303 (7,573) 5,104
Total expense accrual 24,596 18,851 47,646 39,003
Less: claim payments related to:        
Current period 2,979 2,739 3,755 3,138
Prior periods 14,271 13,741 30,262 25,803
Total paid 17,250 16,480 34,017 28,941
Workers' compensation claims liabilities, Ending balance 229,611 122,506 229,611 122,506
Incurred but not reported (IBNR) $ 118,959 $ 29,871 $ 118,959 $ 29,871