Quarterly report [Sections 13 or 15(d)]

Significant Accounting Policies - Additional Information (Details)

v3.25.2
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%