Line of credit facility, Reduction of Borrowing Capacity

No definition available.

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Namespace Prefix:	mwa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Total other amounts reducing excess availability.

+ References

No definition available.

+ Details

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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

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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

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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

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=77997519&loc=d3e13433-108611

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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DebtInstrumentIssuanceDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- 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

End 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=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodEndDate
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

+ 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

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

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

- Definition

Date the debt instrument was repurchased, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629

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-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629

+ Details

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

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: 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(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

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- Details

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Period Type:

- Details

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Period Type:

- Details

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- Details

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- Details

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Borrowing Arrangements (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum (Deprecated 2016-01-31)	5.50%			5.50%
Debt Instrument, Fair Value Disclosure	$ 452,300			$ 452,300
Debt Instrument, Redemption Period, End Date				Jun. 15, 2021
Debt Instrument, Redemption Period, Start Date				Jun. 15, 2021
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed				40.00%
Debt Instrument, Redemption Price, Percentage				101.00%
Future maturities of outstanding borrowings
Loss on early extinguishment of debt	6,200	$ 0		$ 6,200	$ 0
Debt Instrument, Issuance Date				Jun. 12, 2018
5.5% Senior Notes	450,000			$ 450,000			$ 0
Debt Instrument, Repurchase Date				Jun. 12, 2018
Domestic Line of Credit [Member]
Revolving credit facility amount	$ (225,000)			$ (225,000)
Potential increase size of the credit facility by an additional amount				$ 150,000
Line of Credit Facility, Interest Rate at Period End	12500.00%			12500.00%
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	$ 18,100			$ 18,100
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses	112,000			112,000
Swing Line Loans [Member]
Revolving credit facility amount	(25,000)			(25,000)
Letters Of Credit Outstanding [Member]
Revolving credit facility amount	(60,000)			(60,000)
Secured Debt [Member]
Long-term Debt, Gross	$ 500,000			$ 500,000
Debt Instrument, Basis Spread on Variable Rate			25000.00%
Debt Instrument, Periodic Payment, Principal			$ 1,225
Agreement termination date			Nov. 25, 2021
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%			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%
Subsequent Event [Member] \| Domestic Line of Credit [Member]
Revolving credit facility amount						$ (175,000)
Line of credit facility, Reduction of Borrowing Capacity						$ 50,000