Workers' Compensation Claims - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
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	380,600,000	$ 451,000,000
Collateral securities maintain to cover potential workers compensation claims losses	74,800,000	85,200,000
Surety bonds and standby letters of credit	74,800,000
Estimated future liability for unsettled workers' compensation	430,046,000	413,397,000	$ 430,736,000	$ 398,838,000	$ 390,814,000	$ 363,517,000
Reserves incurred in excess of retention limits	3,000,000	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	3,000,000	$ 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