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Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Workers Compensation Reserve [Abstract]      
Workers’ compensation claims liabilities, Beginning balance $ 363,517 $ 312,537 $ 255,675
Add: claims expense accrual      
Current period 162,525 154,091 137,852
Prior periods (3,846) 5,159 (301)
Total expense accrual 158,679 159,250 137,551
Less: claim payments related to      
Current period 23,444 19,537 20,180
Prior periods 85,603 82,573 69,626
Total paid 109,047 102,110 89,806
Add: change in claims incurred in excess of retention limits 248 (6,160) 9,117
Workers' compensation claims liabilities, Ending balance 413,397 363,517 312,537
Incurred but not reported (IBNR) $ 260,529 $ 202,227 $ 158,169
Ratio of IBNR to workers' compensation claims liabilities 63.00% 56.00% 51.00%