Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Rent expense	$ 3,600,000	$ 3,500,000	$ 3,500,000
Health compensation plan annual stop-loss insurance per claim	200,000
Workers compensation plan annual stop-loss insurance per claim	250,000
Accruals for health and workers' compensation plans	$ 2,900,000	$ 2,400,000