Income Taxes (Income before Income Tax, Domestic and Foreign) (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Loss before income taxes, U.S.	$ 96.8	$ 55.4	$ 71.6
Loss before income taxes, Non-U.S.	0.2	(4.7)	1.9
Net Income Before Income Tax	$ 97.0	$ 50.7	$ 73.5

Net income attributable to Parent before income taxes and discontinued operations

No definition available.

Name:	mwa_NetIncomeBeforeIncomeTaxandDiscontinuedOperations
Namespace Prefix:	mwa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The portion of earnings or loss from continuing operations before income taxes that is attributable to domestic operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The portion of earnings or loss from continuing operations before income taxes that is attributable to foreign operations, which is defined as Income or Loss generated from operations located outside the entity's country of domicile.

+ References

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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