Workers' Compensation Claims - Summarizes Aggregate Workers' Compensation Reserve Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Workers Compensation Reserve [Abstract]
Workers' compensation claims liabilities, Beginning balance	$ 413,397	$ 363,517	$ 312,537
Add: claims expense accrual
Current period	161,691	162,525	154,091
Prior periods	(13,355)	(3,846)	5,159
Total expense accrual	148,336	158,679	159,250
Less: claim payments related to
Current period	24,414	23,444	19,537
Prior periods	98,445	85,603	82,573
Total paid	122,859	109,047	102,110
Change in claims incurred in excess of retention limits	112	248	(6,160)
Workers' compensation claims liabilities, Ending balance	438,986	413,397	363,517
Incurred but not reported (IBNR)	$ 285,191	$ 260,529	$ 202,227
Ratio of IBNR to workers' compensation claims liabilities	65.00%	63.00%	56.00%