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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Apr. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash, cash equivalents, and restricted cash |  |  | $ 24,039,000 | $ 21,389,000 |  |  |  | $ 14,781,000 | $ 52,708,000
Cash and cash equivalents |  |  | 23,789,000 |  |  |  |  | 14,531,000 |
Restricted cash |  |  | 250,000 |  |  |  |  | 250,000 |
Captive insurance collateral, debt securities | [1] |  | 50,253,000 |  |  |  |  | 45,419,000 |
Accrued expenses and other current liabilities |  |  | 127,610,000 |  |  |  |  | 116,436,000 |
Other long-term liabilities |  |  | $ 24,585,000 |  |  |  |  | 24,012,000 |
Operating lease expiration year |  |  | 2033 |  |  |  |  |  |
Undiscounted future minimum lease payments through 2032 |  |  | $ 131,300,000 |  |  |  |  |  |
ASU 2014-09 |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Revenue, practical expedient, financing component [true false] |  |  | true |  |  |  |  |  |
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 |  |  | 17,100,000 |  |  |  |  |  |
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 |  |  |  |  |  |
Charge on weather hedge contract |  |  |  | $ 3,100,000 |  |  |  |  |
Payments of derivative hedging contract |  | $ 1,900,000 |  |  |  |  |  |  |
Subsidiaries of Swiss Re | Delivery and branch expenses |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Charge on weather hedge contract |  |  | 2,000,000 |  |  |  |  |  |
Subsidiaries of Swiss Re | Scenario Forecast |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Derivative maximum payout |  |  |  |  | $ 5,000,000 | $ 5,000,000 | $ 5,000,000 |  |
Level 1 |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Captive insurance collateral, debt securities |  |  | 49,700,000 |  |  |  |  | 44,800,000 |
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 |  |  | 17,800,000 |  |  |  |  |  |
Mutual Funds |  |  |  |  |  |  |  |  |
Schedule Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Captive insurance collateral, net asset value |  |  | $ 600,000 |  |  |  |  | $ 600,000 |
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 | $ 12,500,000 | $ 12,500,000 |  |
[1] | See Note 2 – Investments

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