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Significant Accounting Policies - Additional Information (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024 | Sep. 30, 2024
Summary of Account Policies [Line Items] |  |  |  |  |
Restricted cash | $ 1,969,000 |  | $ 1,969,000 |  | $ 1,998,000
Restricted investments | 21,954,000 |  | 21,954,000 |  | $ 18,020,000
Debt securities, available-for-sale, allowance for credit loss | $ 0 | $ 0 | $ 0 | $ 0 |
Department of Transportation | Revenues | Customer Concentration Risk |  |  |  |  |
Summary of Account Policies [Line Items] |  |  |  |  |
Concentration risk percentage | 46.10% | 42.30% | 40.80% | 39.70% |

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