Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Workers Compensation Reserve [Abstract]
Workers’ compensation claims liabilities, Beginning balance	$ 326,233	$ 265,142	$ 312,537	$ 255,675
Add: claims expense accrual
Current period	37,878	34,627	73,153	65,781
Prior periods	2,350	(1,396)	5,264	(548)
Total expense accrual	40,228	33,231	78,417	65,233
Less: claim payments related to
Current period	4,148	3,698	5,280	4,969
Prior periods	25,998	17,625	49,462	38,889
Total paid	30,146	21,323	54,742	43,858
Add: Change in claims incurred in excess of retention limits	(6,236)		(6,133)
Workers’ compensation claims liabilities, Ending balance	330,079	277,050	330,079	277,050
Incurred but not reported (IBNR)	$ 167,693	$ 140,115	$ 167,693	$ 140,115