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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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Number of aircraft.

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- Definition

Amount of allowance for credit loss on accounts receivable.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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