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Concentrations - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018
Concentration Risk [Line Items] |  |  |
Allowance for doubtful accounts | $ 1.3 |  | $ 1.3
Sales Revenue, Net [Member] | American Airlines Inc. [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 53.00% | 53.00% |
Sales Revenue, Net [Member] | United [Member] | Customer Concentration Risk [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 47.00% | 47.00% |

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