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Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Concentration Risk [Line Items]
Restricted cash	$ 3,351		$ 3,351		$ 3,446
Outstanding letters of credit to be collateralized by amounts on deposit	3,400		3,400
Allowance for doubtful accounts	$ 1,300		$ 1,300		$ 800
Sales Revenue, Net [Member] | American Airlines Inc. [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	45.00%	51.00%	46.00%	51.00%
Sales Revenue, Net [Member] | United [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	53.00%	49.00%	52.00%	49.00%

X

- Definition

Amount of allowance for credit loss on accounts receivable.

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- Definition

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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Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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