debt instrument, change in spread on variable rate

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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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

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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 FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Amount of the required periodic payments applied to principal.

+ References

+ Details

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

- Definition

Amount, after accumulated amortization, of debt discount.

+ 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=64930785&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=64930739&loc=d3e28878-108400

+ Details

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

- Definition

Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

+ 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

- References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_DomesticLineOfCreditMember
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Balance Type:
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- Details

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Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=mwa_LettersOfCreditOutstandingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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- Details

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- Details

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Borrowing Arrangements Narrative (Details) - USD ($) $ in Thousands	6 Months Ended	9 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Sep. 30, 2016
Future maturities of outstanding borrowings
Debt Instrument, Unamortized Discount		$ 1,600
Domestic Line of Credit [Member]
Revolving credit facility amount		(225,000)
Potential increase size of the credit facility by an additional amount		$ 150,000
Line of Credit Facility, Interest Rate at Period End		15000.00%
Agreement termination date		Jul. 13, 2021
Aggregate commitments availability		$ 17,500
Aggregate commitments availability, percentage		10.00%
Outstanding letter of credit accrued fees and expenses		$ 23,500
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses		109,500
Swing Line Loans [Member]
Revolving credit facility amount		(25,000)
Letters Of Credit Outstanding [Member]
Revolving credit facility amount		(60,000)
Secured Debt [Member]
Long-term Debt, Gross		500,000	$ 150,000
debt instrument, change in spread on variable rate	7500.00%
Debt Instrument, Basis Spread on Variable Rate	25000.00%
Debt Instrument, Interest Rate, Effective Percentage	4.36%		4.841%
Debt Instrument, Periodic Payment, Principal	$ 1,225
Agreement termination date	Nov. 25, 2021
Future maturities of outstanding borrowings
Financial Liabilities Fair Value Disclosure		$ 490,600
Minimum [Member] \| Domestic Line of Credit [Member]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		2500.00%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member] \| Long-term Debt [Member]
Debt Instrument, Basis Spread on Variable Rate		2500.00%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member] \| Domestic Line of Credit [Member]
Debt Instrument, Basis Spread on Variable Rate		12500.00%
London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member] \| Secured Debt [Member]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum (Deprecated 2016-01-31)		0.75%
London Interbank Offered Rate (LIBOR) [Member] \| Maximum [Member] \| Domestic Line of Credit [Member]
Debt Instrument, Basis Spread on Variable Rate		15000.00%
Base Rate [Member] \| Maximum [Member] \| Domestic Line of Credit [Member]
Debt Instrument, Basis Spread on Variable Rate		5000.00%