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Derivative Financial Instruments (Details) - USD ($) $ in Millions		9 Months Ended	60 Months Ended
	Apr. 07, 2015	Jun. 30, 2015	Sep. 30, 2021	Sep. 30, 2016
Derivative [Line Items]
Derivative, Inception Date	Apr. 07, 2015
Cash Flow Hedge Derivative Instrument Assets at Fair Value		$ 0.9
Secured Debt [Member]
Derivative [Line Items]
Long-term Debt, Gross		$ 500.0
Debt Instrument, Interest Rate, Effective Percentage		4.00%
Debt Instrument, Basis Spread on Variable Rate		32500.00%
Scenario, Forecast [Member]
Derivative [Line Items]
HedgePeriodStart			Sep. 30, 2016
Derivative, Cap Interest Rate				0.75%
Derivative, Fixed Interest Rate				2.341%
Derivative, Amount of Hedged Item				$ 150.0
Derivative, Maturity Date			Sep. 30, 2021
Scenario, Forecast [Member] | Secured Debt [Member]
Derivative [Line Items]
Long-term Debt, Gross				$ 150.0
Debt Instrument, Interest Rate, Effective Percentage				5.591%

X

- Definition

Date on which a hedging relationship begins.

+ References

No definition available.

+ Details

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Namespace Prefix:	mwa_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Fair value of all asset derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 815



-SubTopic 10



-Section 50



-Paragraph 4B



-Subparagraph (c)(1)



-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	duration

X

- Definition

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 210



-SubTopic 10



-Section S99



-Paragraph 1



-Subparagraph (SX 210.5-02.22)



-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef



-Publisher SEC



-Name Regulation S-X (SX)



-Number 210



-Section 02



-Paragraph 22



-Article 5



Reference 3: http://www.xbrl.org/2003/role/presentationRef



-Publisher SEC



-Name Regulation S-X (SX)



-Number 210



-Section 02



-Paragraph 20



-Article 5



Reference 4: http://www.xbrl.org/2003/role/presentationRef



-Publisher SEC



-Name Regulation S-X (SX)



-Number 210



-Section 03



-Paragraph 16



-Article 9

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

X

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 210



-SubTopic 10



-Section S99



-Paragraph 1



-Subparagraph (SX 210.5-02.22(a)(1))



-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 835



-SubTopic 30



-Section 45



-Paragraph 2



-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 3: http://www.xbrl.org/2003/role/presentationRef



-Publisher SEC



-Name Regulation S-X (SX)



-Number 210



-Section 02



-Paragraph 22



-Article 5

+ Details

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Balance Type:	na
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X

- Definition

Amount of the hedged item as of the balance sheet date related to the derivative. For example, the hedged balance on a debt instrument.

+ References

No definition available.

+ Details

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X

- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

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X

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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X

- Definition

Date the entity entered into the derivative contract, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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- 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

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X

- Definition

Date the derivative contract matures, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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- Details

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X

- Details

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