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Commitments - Additional Information (Detail) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) Aircraft | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) Aircraft | Jun. 30, 2017 USD ($)
Operating Leased Assets [Line Items] |  |  |  |
Number of leased aircraft | Aircraft | 28 |  | 28 |
Aggregate rental expense under all operating aircraft, equipment and facility leases | $ | $ 21.6 | $ 21.0 | $ 62.8 | $ 62.3
Aircraft [Member] |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Operating leases remaining term | 5 years 9 months |  | 5 years 9 months |
Headquarters and other facility [Member] |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Operating leases remaining term | 9 years |  | 9 years |

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- Definition Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
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- Definition 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. + References No definition available. + Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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- Details
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- Details
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