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Concentrations of Credit Risk - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2022 | Mar. 31, 2021 | Mar. 31, 2022 | Mar. 31, 2021 | Sep. 30, 2021
Concentration Risk [Line Items] |  |  |  |  |
Restricted cash | $ 3,351,000 |  | $ 3,351,000 |  | $ 3,350,000
Outstanding letters of credit to be collateralized by amounts on deposit | 3,400,000 |  | 3,400,000 |  |
Allowance for doubtful accounts | $ 0 |  | $ 0 |  | $ 0
Sales Revenue, Net [Member] | American Airlines Inc. [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 46.00% | 45.00% | 46.00% | 46.00% |
Sales Revenue, Net [Member] | United [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 47.00% | 53.00% | 48.00% | 52.00% |

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