Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Workers Compensation Reserve [Abstract]
Workers’ compensation claims liabilities, Beginning balance	$ 378,874	$ 326,233	$ 363,517	$ 312,537
Add: claims expense accrual
Current period	39,967	37,878	79,019	73,153
Prior periods		2,350	(6)	5,264
Total expense accrual	39,967	40,228	79,013	78,417
Less: claim payments related to
Current period	5,148	4,148	6,281	5,280
Prior periods	22,856	25,998	45,391	49,462
Total paid	28,004	30,146	51,672	54,742
Add: Change in claims incurred in excess of retention limits	(23)	(6,236)	(44)	(6,133)
Workers' compensation claims liabilities, Ending balance	390,814	330,079	390,814	330,079
Incurred but not reported (IBNR)	$ 231,702	$ 167,693	$ 231,702	$ 167,693
Ratio of IBNR to workers' compensation claims liabilities	59.00%	51.00%	59.00%	51.00%