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Concentrations - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Sep. 30, 2018
Concentration Risk [Line Items] |  |  |  |  |
Allowance for doubtful accounts | $ 1.4 |  | $ 1.4 |  | $ 1.3
Sales Revenue, Net [Member] | American Airlines Inc. [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 55.00% | 54.00% | 54.00% | 54.00% |
Sales Revenue, Net [Member] | United [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 45.00% | 46.00% | 46.00% | 46.00% |

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