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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Apr. 30, 2020 | Apr. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Sep. 30, 2021 | Sep. 30, 2020 | Oct. 01, 2019 | Sep. 30, 2019 | Sep. 30, 2018
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash, cash equivalents, and restricted cash |  |  | $ 66,968,000 | $ 5,967,000 |  |  |  | $ 5,149,000 | $ 14,781,000
Cash and cash equivalents |  |  | 66,718,000 |  |  |  |  | 4,899,000 |
Restricted cash |  |  | 250,000 |  |  |  |  | 250,000 |
Captive insurance collateral, debt securities |  |  | 69,607,000 |  |  |  |  | 58,490,000 |
Additional payment obligated to pay if degree days exceed ten year average |  |  | 5,000,000.0 |  |  |  |  |  |
Accrued expenses and other current liabilities |  |  | 153,591,000 |  |  |  |  | 120,839,000 |
Other long-term liabilities |  |  | 22,235,000 |  |  |  |  | 25,746,000 |
Operating lease right-of-use assets |  |  | 100,765,000 |  |  |  |  |  |
Current portion of operating lease liabilities |  |  | 19,391,000 |  |  |  |  |  |
Long-term operating lease liabilities |  |  | 86,680,000 |  |  |  |  |  |
Interest rate swap |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Notional Value |  |  | 65,300,000 |  |  |  |  | 45,000,000.0 |
ASC Topic 842 |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Operating lease right-of-use assets |  |  |  |  |  |  | $ 104,700,000 |  |
Current portion of operating lease liabilities |  |  |  |  |  |  | 20,100,000 |  |
Long-term operating lease liabilities |  |  |  |  |  |  | $ 89,900,000 |  |
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 |  |  | 16,800,000 |  |  |  |  |  |
Subsidiaries of Swiss Re |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Payments and receivables of derivative hedging contract | $ (10,100,000) | $ 2,100,000 |  |  |  |  |  |  |
Delivery and branch expenses | Financial Products Corporation |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Change on weather hedge contract |  |  | (10,100,000) | $ 2,100,000 |  |  |  |  |
Scenario Forecast |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Derivative maximum payout |  |  |  |  | $ 5,000,000.0 | $ 5,000,000.0 |  |  |
Significant Other Observable Inputs Level 2 | New England Teamsters & Trucking Industry Pension Fund |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Multiemployer plan discounted withdrawal liability |  |  | 29,300,000 |  |  |  |  | 21,100,000 |
Captive Insurance Collateral | Level 1 |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Captive insurance collateral, debt securities |  |  | 68,800,000 |  |  |  |  | 58,000,000.0 |
Captive Insurance Collateral | Mutual Funds |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Captive insurance collateral, net asset value |  |  | $ 800,000 |  |  |  |  | $ 500,000 |
Maximum |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash equivalents, highly liquid investments maturity |  |  | 3 months |  |  |  |  |  |
Maximum | Scenario Forecast |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Derivative maximum receivable |  |  |  |  | $ 12,500,000 | $ 12,500,000 |  |  |

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