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Leases and Commitments - Additional Information (Detail) $ in Thousands | 3 Months Ended |
--- | --- | ---
Dec. 31, 2019 USD ($) Air-craft | Dec. 31, 2018 USD ($) | Oct. 01, 2019 USD ($)
Operating Leased Assets [Line Items] |  |  |
Operating lease, right-of-use assets | $ 146,071 |  |
Number of leased aircraft | Air-craft | 18 |  |
Aggregate rental expense under all operating aircraft, equipment and facility leases | $ 16,800 | $ 19,100 |
Operating lease current liabilities | 37,674 |  |
Operating lease noncurrent liabilities | 95,992 |  |
Operating lease payments in operating cash flows | $ 9,534 |  |
Accounting Standards Update 2016-02 [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Operating lease, right-of-use assets |  |  | $ 154,600
Operating lease, liability |  |  | 141,900
Accounting Standards Update 2016-02 [Member] | Prepaid Aircraft Rent [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Operating lease, right-of-use assets |  |  | 35,800
Accounting Standards Update 2016-02 [Member] | Deferred Rent Credits [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Operating lease, right-of-use assets |  |  | 21,300
Accounting Standards Update 2016-02 [Member] | Accrued Aircraft Rents [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Operating lease, right-of-use assets |  |  | $ 1,800

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