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Commitments - Additional Information (Detail) $ in Millions	3 Months Ended
	Dec. 31, 2018 USD ($) Air-craft	Dec. 31, 2017 USD ($)
Operating Leased Assets [Line Items]
Number of leased aircraft | Air-craft	28
Aggregate rental expense under all operating aircraft, equipment and facility leases | $	$ 19.1	$ 21.7
Aircraft [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	5 years 3 months
Headquarters and other facility [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	7 years

X

- Definition

Number of leased aircraft.

+ References

No definition available.

+ Details

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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

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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.

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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

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- Details

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