Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule Of Significant Accounting Policies [Line Items]
Cash, cash equivalents, and restricted cash	$ 21,389,000			$ 52,708,000	$ 37,326,000	$ 139,188,000
Cash and cash equivalents	21,139,000			52,458,000
Restricted cash	250,000			250,000
Captive insurance collateral, debt securities	45,100,000			11,300,000
Cash deposit in support of liability	34,200,000
Accrued expenses and other current liabilities	119,681,000			108,449,000
Other long-term liabilities	$ 23,037,000			23,659,000
Operating lease expiration year	2032
Undiscounted future minimum lease payments through 2032	$ 131,100,000
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	17,300,000
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	22,300,000
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	$ 5,000,000
Subsidiaries of Swiss Re | Delivery and branch expenses
Schedule Of Significant Accounting Policies [Line Items]
Charge on weather hedge contract	3,100,000
Credit 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		$ 12,500,000	$ 17,500,000