Workers' Compensation Claims - Additional Information (Detail) - USD ($)		9 Months Ended	12 Months Ended
	Jul. 01, 2018	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Worker's compensation risk claim maximum	$ 5,000,000	$ 5,000,000
Balance in Chubb trust accounts		430,000,000	$ 380,600,000
Surety bonds	30,000,000
Specified investment balances or other financial instruments		85,200,000	96,800,000
Surety bonds and standby letters of credit		85,200,000
Estimated future liability for unsettled workers' compensation		398,838,000	363,517,000	$ 390,814,000	$ 344,685,000	$ 330,079,000	$ 312,537,000
Reserves incurred in excess of retention limits		2,900,000	3,000,000
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		2,900,000	$ 3,000,000
California [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Surety bonds and standby letters of credit		70,600,000
Wells Fargo Bank, National Association [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Letter of credit	$ 63,700,000
Chubb Limited [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Worker's compensation risk claim maximum		$ 5,000,000