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Concentrations - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Concentration Risk [Line Items]
Allowance for doubtful accounts	$ 0.8	$ 1.0
American Airlines Inc. [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	52.00%	53.00%	54.00%
United [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	48.00%	47.00%	46.00%

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- Definition

Amount of allowance for credit loss on accounts receivable.

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- Definition

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