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Income Taxes (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Income Tax Disclosure [Abstract]
Transition Tax Liability	$ 1.7
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	4.7
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 0.4	$ 0.4

X

- Definition

Transition Tax Liability

+ References

No definition available.

+ Details

Name:	mwa_TransitionTaxLiability
Namespace Prefix:	mwa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-6

+ Details

Name:	us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

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

X

- Definition

Amount of interest expense accrued for an underpayment of income taxes.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-15

+ Details

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Balance Type:	credit
Period Type:	instant