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Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Workers Compensation Reserve [Abstract]        
Workers' compensation claims liabilities, Beginning balance   $ 225,278 $ 121,056 $ 78,511
Claims expense accrual:        
Current year   122,740 104,314 69,708
Prior years $ 61,300 (13,683) 66,719 22,457
Total expense accrual $ 115,600 109,057 171,033 92,165
Claim payments related to:        
Current year   17,517 15,243 12,418
Prior years   61,143 51,568 37,202
Total paid   78,660 66,811 49,620
Workers' compensation claims liabilities, Ending balance   255,675 225,278 121,056
Incurred but not reported (IBNR)   $ 127,792 $ 113,984 $ 69,611