Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended		12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2019	Sep. 30, 2017	Sep. 30, 2016
Schedule Of Significant Accounting Policies [Line Items]
Cash, cash equivalents, and restricted cash		$ 35,679,000	$ 53,145,000		$ 52,708,000	$ 139,188,000
Cash and cash equivalents		35,429,000			52,458,000
Restricted cash		250,000			250,000
Captive insurance collateral, debt securities		43,900,000			11,300,000
Cash deposit in support of liability	$ 34,200,000
Accrued expenses and other current liabilities		122,564,000			108,449,000
Other long-term liabilities		$ 22,613,000			23,659,000
Operating lease expiration year		2032
Undiscounted future minimum lease payments through 2032		$ 128,200,000
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		21,800,000
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				$ 5,000,000
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		$ 1,300,000			$ 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				$ 12,500,000