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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 |

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