Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Workers Compensation Reserve [Abstract]
Workers’ compensation claims liabilities, Beginning balance	$ 413,397	$ 363,517
Add: claims expense accrual
Current period	40,385	39,052
Prior periods	(1,700)	(6)
Total expense accrual	38,685	39,046
Less: claim payments related to
Current period	1,459	1,133
Prior periods	27,716	22,535
Total paid	29,175	23,668
Add: Change in claims incurred in excess of retention limits	(35)	(21)
Workers' compensation claims liabilities, Ending balance	422,872	378,874
Incurred but not reported (IBNR)	$ 268,291	$ 217,492
Ratio of IBNR to workers' compensation claims liabilities	63.00%	57.00%