Borrowing Arrangements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2013
Line of Credit Facility, Interest Rate at Period End	17500.00%
Agreement termination date	Dec. 18, 2017
Debt Instrument, Redemption Period, Start Date	Aug. 31, 2015
Swing Line Loans [Member]
Revolving credit facility amount	$ 25
Letters Of Credit Outstanding [Member]
Revolving credit facility amount	60
Eight Point Seven Five Senior Unsecured Notes [Member]
Senior subordinated notes bear interest	8.75%
Debt Instrument, Maturity Date	Sep. 01, 2020
Unamortized discount	2.0
Subordinated notes fair value	202.1
Seven Point Three Seven Five Senior Subordinated Notes [Member]
Senior subordinated notes bear interest	7.375%
Debt Instrument, Maturity Date	Jun. 01, 2017
Subordinated notes fair value	431.6
ABL Agreement [Member]
Revolving credit facility amount	225
Potential increase size of the credit facility by an additional amount	150
Aggregate commitments availability	22.5
Aggregate commitments availability, percentage	10.00%
Outstanding letter of credit accrued fees and expenses	30.8
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses	$ 138.5
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	3750.00%
Minimum [Member]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	2500.00%
Maximum [Member]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	3750.00%
Change in Control [Member] \| Eight Point Seven Five Senior Unsecured Notes [Member]
Debt Instrument, Redemption Price, Percentage	101.00%
Change in Control [Member] \| Seven Point Three Seven Five Senior Subordinated Notes [Member]
Debt Instrument, Redemption Price, Percentage	101.00%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member]
Borrowing rate in addition to LIBOR	17500.00%
London Interbank Offered Rate (LIBOR) [Member] \| Maximum [Member]
Borrowing rate in addition to LIBOR	22500.00%
Base Rate [Member] \| Minimum [Member]
Borrowing rate in addition to LIBOR	7500.00%
Base Rate [Member] \| Maximum [Member]
Borrowing rate in addition to LIBOR	12500.00%

Maximum excess availability level at which financial maintenance covenants would apply.

No definition available.

Name:	mwa_MaximumExcessAvailabilityLevelAtWhichFinancialMaintenanceCovenantsWouldApply
Namespace Prefix:	mwa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Maximum excess availability level at which financial maintenance covenants would apply, as a percentage of aggregate commitments.

+ References

No definition available.

+ Details

Name:	mwa_MaximumExcessAvailabilityLevelAtWhichFinancialMaintenanceCovenantsWouldApplyAsPercentageOfAggregateCommitments
Namespace Prefix:	mwa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Total other amounts reducing excess availability.

+ References

No definition available.

+ Details

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

- Definition

Potential increase in the size of credit facility.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13476-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: 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

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: 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)(2))

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

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Start date of debt instrument redemption period, in CCYY-MM-DD format.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 470

-Section 50

-Paragraph 3

-Subparagraph e

-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodStartDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

+ Details

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

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ 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 210.5-02.19(b),22(b))

-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 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration