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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items] |  |
Engine overhaul expense | $ 10.6 | $ 4.1
Engine overhaul pass-through expense | 1.9 | 1.5
Airframe check expense | 7.3 | 1.5
Airframe check pass-through expense | $ 1.2 | $ 0.0
Aircraft [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Percentage leased | 12.00% |

X
- Definition Airframe check expense. + References No definition available. + Details
Name: | mesa_AirframeCheckExpense
Namespace Prefix: | mesa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Airframe check, pass-through expense. + References No definition available. + Details
Name: | mesa_AirframeCheckPassThroughExpense
Namespace Prefix: | mesa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Engine overhaul expense. + References No definition available. + Details
Name: | mesa_EngineOverhaulExpense
Namespace Prefix: | mesa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Engine overhaul, pass-through expense. + References No definition available. + Details
Name: | mesa_EngineOverhaulPassThroughExpense
Namespace Prefix: | mesa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage leased. + References No definition available. + Details
Name: | mesa_PercentageLeased
Namespace Prefix: | mesa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Summary Of Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | mesa_SummaryOfSignificantAccountingPoliciesLineItems
Namespace Prefix: | mesa_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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