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Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Sep. 30, 2020
Concentration Risk [Line Items] |  |  |  |  |
Restricted cash | $ 3,352 |  | $ 3,352 |  | $ 3,446
Outstanding letters of credit to be collateralized by amounts on deposit | 3,400 |  | 3,400 |  |
Allowance for doubtful accounts | $ 1,400 |  | $ 1,400 |  | $ 800
Sales Revenue, Net [Member] | American Airlines Inc. [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 45.00% | 60.00% | 46.00% | 53.00% |
Sales Revenue, Net [Member] | United [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 51.00% | 40.00% | 52.00% | 47.00% |

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