Workers' Compensation Claims - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Worker's compensation risk claim maximum	$ 5,000,000
Surety bonds	15,000,000
Balance in Chubb trust accounts	393,500,000	$ 451,000,000
Collateral securities maintain to cover potential workers compensation claims losses	76,100,000	85,200,000
Surety bonds and standby letters of credit	76,100,000
Estimated future liability for unsettled workers' compensation	438,986,000	413,397,000	$ 363,517,000	$ 312,537,000
Reserves incurred in excess of retention limits	3,300,000	3,200,000
Other Assets [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accrual for costs incurred in excess of retention limits offset by a receivable	3,300,000	$ 3,200,000
Wells Fargo Bank, National Association [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Letter of credit	63,700,000
Maryland [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reinsurance coverage	1,000,000
Colorado [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reinsurance coverage	2,000,000
California [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Surety bonds and standby letters of credit	$ 55,600,000