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Concentrations - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Sep. 30, 2017
Concentration Risk [Line Items] |  |  |  |  |
Allowance for doubtful accounts | $ 1.7 |  | $ 1.7 |  | $ 0.7
American Airlines Inc. [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 54.00% | 55.00% | 57.00% | 57.00% |
United Airlines, Inc. [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Concentration risk, percentage | 46.00% | 45.00% | 43.00% | 43.00% |

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