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Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Workers Compensation Reserve [Abstract]        
Workers' compensation claims liabilities, Beginning balance $ 315,733 $ 336,504 $ 357,746 $ 438,986
Add: claims expense accrual        
Current period 2,394 35,138 71,407 106,359
Prior periods (823) (2,977) (7,486) (5,163)
Total expense accrual 1,571 32,161 63,921 101,196
Less: claim payments related to        
Current period 6,149 7,046 11,129 13,123
Prior periods 14,018 15,640 113,022 181,170
Total paid 20,167 22,686 124,151 194,293
Change in claims incurred in excess of retention limits (138) 1,369 (517) 1,459
Workers' compensation claims liabilities, Ending balance 296,999 347,348 296,999 347,348
Incurred but not reported (IBNR) $ 162,382 $ 203,595 $ 162,382 $ 203,595
Ratio of IBNR to workers' compensation claims liabilities 55.00% 59.00% 55.00% 59.00%