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Concentrations - Additional Information (Detail) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 USD ($) Air-craft | Sep. 30, 2018 USD ($) | Sep. 30, 2017
Concentration Risk [Line Items] |  |  |
Number of aircraft | Air-craft | 17 |  |
Allowance for doubtful accounts | $ | $ 1.0 | $ 1.3 |
American Airlines Inc. [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 53.00% | 54.00% | 56.00%
United [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 47.00% | 46.00% | 44.00%

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