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Recent Accounting Pronouncements - Additional Information (Detail) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2019 USD ($) Air-craft | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Number of leased aircraft | Air-craft |  | 18 |  |
Deferred income taxes |  | $ 15,503 | $ (17,891) | $ 20,515
Income tax expense (benefit) |  | $ 15,706 | $ (17,426) | $ 20,874
Accounting Standards Update 2016-02 |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Number of leased aircraft | Air-craft |  | 18 |  |
Number of nominal operating leased aircraft | Air-craft |  | 42 |  |
Accounting Standards Update 2016-02 | Minimum [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Operating lease, liability |  | $ 130,000 |  |
Operating lease, right-of-use asset |  | 120,000 |  |
Accounting Standards Update 2016-02 | Maximum [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Operating lease, liability |  | 170,000 |  |
Operating lease, right-of-use asset |  | $ 180,000 |  |
Accounting Standards Update 2016-09 [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Deferred income taxes | $ 400 |  |  |
Income tax expense (benefit) | 300 |  |  |
Cumulative-effect adjustment increasing retained earnings | $ 700 |  |  |

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