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Borrowing Arrangements (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended | 0 Months Ended |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Mar. 31, 2015 | Mar. 31, 2014 | Apr. 07, 2015 | Sep. 30, 2014 | Sep. 30, 2013
Borrowing rate in addition to LIBOR |  |  | 32500.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 |  |  |  |
Cash and cash equivalents | $ 34.1 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 105.1 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 34.1 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 105.1 us-gaap_CashAndCashEquivalentsAtCarryingValue |  | $ 161.1 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 123.6 us-gaap_CashAndCashEquivalentsAtCarryingValue
Loss on early extinguishment of debt | 0 us-gaap_GainsLossesOnExtinguishmentOfDebt | 0 us-gaap_GainsLossesOnExtinguishmentOfDebt | 31.3 us-gaap_GainsLossesOnExtinguishmentOfDebt | 0 us-gaap_GainsLossesOnExtinguishmentOfDebt |  |  |
Gain (Loss) on Repurchase of Debt Instrument |  |  | (25.2)us-gaap_GainLossOnRepurchaseOfDebtInstrument |  |  |  |
Write off of Deferred Debt Issuance Cost |  |  | 4.4 us-gaap_WriteOffOfDeferredDebtIssuanceCost |  |  |  |
Write off of Unamortized Discount |  |  | 1.7 mwa_WriteoffofUnamortizedDiscount |  |  |  |
Payments of Debt Issuance Costs |  |  | 8.4 us-gaap_PaymentsOfDebtIssuanceCosts | 0 us-gaap_PaymentsOfDebtIssuanceCosts |  |  |
Debt Instrument, Unamortized Discount | 2.4 us-gaap_DebtInstrumentUnamortizedDiscount |  | 2.4 us-gaap_DebtInstrumentUnamortizedDiscount |  |  |  |
Domestic Line of Credit [Member] |  |  |  |  |  |  |
Revolving credit facility amount | (225)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  | (225)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Potential increase size of the credit facility by an additional amount |  |  | 150 mwa_PotentialIncreaseInSizeOfCreditFacility / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Line of Credit Facility, Interest Rate at Period End | 17500.00%us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  | 17500.00%us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Agreement termination date |  |  | Dec. 18, 2017 |  |  |  |
Aggregate commitments availability |  |  | 22.5 mwa_MaximumExcessAvailabilityLevelAtWhichFinancialMaintenanceCovenantsWouldApply / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Aggregate commitments availability, percentage |  |  | 10.00%mwa_MaximumExcessAvailabilityLevelAtWhichFinancialMaintenanceCovenantsWouldApplyAsPercentageOfAggregateCommitments / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Outstanding letter of credit accrued fees and expenses | 42.4 mwa_OutstandingLetterOfCreditAccruedFeesAndExpenses / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  | 42.4 mwa_OutstandingLetterOfCreditAccruedFeesAndExpenses / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Excess availability reduced by outstanding borrowings, outstanding letters of credit and accrued fees and expenses | 172.8 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  | 172.8 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 3750.00%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember |  |  |  |
Swing Line Loans [Member] |  |  |  |  |  |  |
Revolving credit facility amount | (25)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = mwa_SwingLineLoansMember |  | (25)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = mwa_SwingLineLoansMember |  |  |  |
Letters Of Credit Outstanding [Member] |  |  |  |  |  |  |
Revolving credit facility amount | (60)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = mwa_LettersOfCreditOutstandingMember |  | (60)us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = mwa_LettersOfCreditOutstandingMember |  |  |  |
Secured Debt [Member] |  |  |  |  |  |  |
Long-term Debt, Gross | 500.0 us-gaap_DebtInstrumentCarryingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember |  | 500.0 us-gaap_DebtInstrumentCarryingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember |  |  |  |
Debt Instrument, Periodic Payment, Principal |  |  | 1.25 us-gaap_DebtInstrumentPeriodicPaymentPrincipal / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember |  |  |  |
Agreement termination date |  |  | Nov. 25, 2021 |  |  |  |
Financial Liabilities Fair Value Disclosure | $ 501.2 us-gaap_FinancialLiabilitiesFairValueDisclosure / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember |  | $ 501.2 us-gaap_FinancialLiabilitiesFairValueDisclosure / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember |  |  |  |
Minimum [Member] | Domestic Line of Credit [Member] |  |  |  |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 2500.00%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |  |
Maximum [Member] | Domestic Line of Credit [Member] |  |  |  |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 3750.00%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Secured Debt [Member] |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum |  |  | 0.75%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Long-term Debt [Member] |  |  |  |  |  |  |
Borrowing rate in addition to LIBOR | 7500.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Domestic Line of Credit [Member] |  |  |  |  |  |  |
Borrowing rate in addition to LIBOR |  |  | 17500.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Domestic Line of Credit [Member] |  |  |  |  |  |  |
Borrowing rate in addition to LIBOR |  |  | 22500.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |  |
Base Rate [Member] | Maximum [Member] | Domestic Line of Credit [Member] |  |  |  |  |  |  |
Borrowing rate in addition to LIBOR |  |  | 12500.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = us-gaap_DomesticLineOfCreditMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |
Derivative, Inception Date |  |  |  |  | Apr. 07, 2015 |  |

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- 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -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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- 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
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