Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Workers Compensation Reserve [Abstract]
Workers' compensation claims liabilities, Beginning balance	$ 422,872	$ 378,874	$ 413,397	$ 363,517
Add: claims expense accrual
Current period	41,145	39,967	81,530	79,019
Prior periods	(2,952)		(4,652)	(6)
Total expense accrual	38,193	39,967	76,878	79,013
Less: claim payments related to
Current period	4,145	5,148	5,604	6,281
Prior periods	26,088	22,856	53,804	45,391
Total paid	30,233	28,004	59,408	51,672
Add: Change in claims incurred in excess of retention limits	(96)	(23)	(131)	(44)
Workers' compensation claims liabilities, Ending balance	430,736	390,814	430,736	390,814
Incurred but not reported (IBNR)	$ 273,114	$ 231,702	$ 273,114	$ 231,702
Ratio of IBNR to workers' compensation claims liabilities	63.00%	59.00%	63.00%	59.00%