Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Workers Compensation Reserve [Abstract]
Workers’ compensation claims liabilities, Beginning balance	$ 312,537	$ 255,675	$ 225,278
Add: claims expense accrual
Current period	154,091	137,852	122,740
Prior periods	5,159	(301)	(13,683)
Total expense accrual	159,250	137,551	109,057
Less: claim payments related to
Current period	19,537	20,180	17,517
Prior periods	82,573	69,626	61,143
Total paid	102,110	89,806	78,660
Add: change in claims incurred in excess of retention limits	(6,160)	9,117
Workers' compensation claims liabilities, Ending balance	363,517	312,537	255,675
Incurred but not reported (IBNR)	$ 202,227	$ 158,169	$ 127,792
Ratio of IBNR to workers' compensation claims liabilities	56.00%	51.00%	50.00%