Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2017	Sep. 30, 2016
Schedule Of Significant Accounting Policies [Line Items]
Cash, cash equivalents, and restricted cash		$ 9,673,000	$ 94,824,000				$ 52,708,000	$ 139,188,000
Cash and cash equivalents		9,423,000					52,458,000
Restricted cash		250,000					250,000
Captive insurance collateral, debt securities		44,500,000					11,300,000
Cash deposit in support of liability	$ 34,200,000
Accrued expenses and other current liabilities		119,815,000					108,449,000
Other long-term liabilities		$ 24,047,000					23,659,000
Operating lease expiration year		2032
Undiscounted future minimum lease payments through 2032		$ 130,300,000
New England Teamsters & Trucking Industry Pension Fund
Schedule Of Significant Accounting Policies [Line Items]
Accrued expenses and other current liabilities		200,000
Other long-term liabilities		17,200,000
New England Teamsters & Trucking Industry Pension Fund | Significant Other Observable Inputs Level 2
Schedule Of Significant Accounting Policies [Line Items]
Multiemployer plan discounted withdrawal liability		21,100,000
Subsidiaries of Swiss Re
Schedule Of Significant Accounting Policies [Line Items]
Additional payment obligated to pay if degree days exceed ten year average		5,000,000
Subsidiaries of Swiss Re | Scenario forecast
Schedule Of Significant Accounting Policies [Line Items]
Derivative maximum payout				$ 5,000	$ 5,000	$ 5,000,000
Subsidiaries of Swiss Re | Delivery and branch expenses
Schedule Of Significant Accounting Policies [Line Items]
Charge on weather hedge contract		1,900,000
Charge or benefit on weather hedge contract			$ 0
Mutual Funds
Schedule Of Significant Accounting Policies [Line Items]
Captive insurance collateral, net asset value		$ 700,000					$ 500,000
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Cash equivalents, highly liquid investments maturity		3 months
Maximum | Subsidiaries of Swiss Re | Scenario forecast
Schedule Of Significant Accounting Policies [Line Items]
Derivative maximum receivable				$ 12,500	$ 12,500	$ 12,500,000