Number of leased aircraft.

No definition available.

Name:	mesa_NumberOfLeasedAircraft
Namespace Prefix:	mesa_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- 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

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- 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

Name:	us-gaap_OperatingLeasedAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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/otherTransitionRef
-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

Name:	us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=mesa_AircraftMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=mesa_HeadquartersAndOtherFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments - Additional Information (Detail) $ in Millions	12 Months Ended
Commitments - Additional Information (Detail) $ in Millions	Sep. 30, 2019 USD ($) Air-craft	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Operating Leased Assets [Line Items]
Number of leased aircraft \| Air-craft	18
Aggregate rental expense under all operating aircraft, equipment and facility leases \| $	$ 72.8	$ 85.9	$ 83.8
Aircraft [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	4 years 6 months
Headquarters and other facility [Member]
Operating Leased Assets [Line Items]
Operating leases remaining term	6 years 2 months 12 days