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Balance Sheet Information - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Balance Sheet Related Disclosures [Abstract]
Depreciation expense	$ 64.6	$ 60.7	$ 45.6
Amortization of unfavorable lease liability	6.6	$ 6.8	$ 6.8
Write off of unfavorable lease liability	$ 1.2

X

- Definition

Amortization of unfavorable lease liability.

+ References

No definition available.

+ Details

Name:	mesa_AmortizationOfUnfavorableLeaseLiability
Namespace Prefix:	mesa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Write off of unfavorable lease liability.

+ References

No definition available.

+ Details

Name:	mesa_WriteOffOfUnfavorableLeaseLiability
Namespace Prefix:	mesa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

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