New spare engines delivery month and year description.

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Number of leased aircraft.

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Number of new spare engines to be acquired.

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Number of new spare engines to be delivered.

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Number of putative class action lawsuits.

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Carrying amount, at the balance sheet date, of deposits made to the manufacturer for new flight equipment still under construction. May include capitalized interest.

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Weighted average discount rate for operating lease calculated at point in time.

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Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

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Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021 USD ($) Air-craft	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Air-craft Engine Lawsuit	Jun. 30, 2020 USD ($)
Purchase Commitment Excluding Longterm Commitment [Line Items]
Number of leased aircraft \| Air-craft	17		17
Aggregate rental expense under all operating aircraft, equipment and facility leases \| $	$ 13.9	$ 15.6	$ 41.6	$ 51.0
Weighted average remaining lease term Operating leases	2 years 10 months 24 days		2 years 10 months 24 days
Weighted average discount rate Operating leases	4.20%		4.20%
IPO [Member]
Purchase Commitment Excluding Longterm Commitment [Line Items]
Number of putative class action lawsuits \| Lawsuit			2
Superior Court of State of Arizona [Member] \| IPO [Member]
Purchase Commitment Excluding Longterm Commitment [Line Items]
Number of putative class action lawsuits \| Lawsuit			1
U.S. District Court of Arizona [Member] \| IPO [Member]
Purchase Commitment Excluding Longterm Commitment [Line Items]
Number of putative class action lawsuits \| Lawsuit			1
CF34-8C5 or CF34-8E5 Engines [Member] \| Letter Agreement No. 13-3 [Member]
Purchase Commitment Excluding Longterm Commitment [Line Items]
Number of new spare engines to be acquired \| Engine			10
New spare engines delivery month and year description			The Company agreed to purchase and take delivery of ten (10) new CF34-8C5 or CF34-8E5 engines with delivery dates starting from July 1, 2021 through November 1, 2022. During the quarter ended March 31, 2021, a $7.0 million non-refundable purchase deposit was made for the first five engines to be delivered in calendar 2021.
Purchase deposit \| $	$ 7.0		$ 7.0
Number of new spare engines to be delivered \| Engine			5
Purchase commitment amount \| $	$ 50.0		$ 50.0