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Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
Concentration Risk [Line Items] |  |  |
Restricted cash | $ 3,350 | $ 3,446 |
Outstanding letters of credit to be collateralized by amounts on deposit | 3,400 | 3,400 |
Allowance for doubtful accounts | $ 300 | $ 800 |
American Airlines Inc. [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 45.00% | 52.00% | 53.00%
United [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 52.00% | 48.00% | 47.00%

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