Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Workers Compensation Reserve [Abstract]
Workers’ compensation claims liabilities, Beginning balance	$ 312,537	$ 255,675
Add: claims expense accrual
Current period	35,275	31,155
Prior periods	2,914	848
Total expense accrual	38,189	32,003
Less: claim payments related to
Current period	1,132	1,271
Prior periods	23,464	21,264
Total paid	24,596	22,535
Add: Change in claims incurred in excess of retention limits	103
Workers’ compensation claims liabilities, Ending balance	326,233	265,143
Incurred but not reported (IBNR)	$ 162,760	$ 132,879