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Stock-based Compensation Plans (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allocated Share-based Compensation Expense	$ 1.9us-gaap_AllocatedShareBasedCompensationExpense	$ 2.0us-gaap_AllocatedShareBasedCompensationExpense	$ 5.3us-gaap_AllocatedShareBasedCompensationExpense	$ 6.1us-gaap_AllocatedShareBasedCompensationExpense
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	7.9us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized		7.9us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	998,980us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	1,087,649us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	1,159,708us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount	2,812,066us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Phantom Share Units (PSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period					289,524us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PhantomShareUnitsPSUsMember
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share	$ 9.85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_PhantomShareUnitsPSUsMember		$ 9.85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_PhantomShareUnitsPSUsMember
Share-based compensation liability	2.3us-gaap_DeferredCompensationSharebasedArrangementsLiabilityCurrentAndNoncurrent / us-gaap_AwardTypeAxis = us-gaap_PhantomShareUnitsPSUsMember		2.3us-gaap_DeferredCompensationSharebasedArrangementsLiabilityCurrentAndNoncurrent / us-gaap_AwardTypeAxis = us-gaap_PhantomShareUnitsPSUsMember
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period					240,691us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value					9.78us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember	$ 8.52us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember	$ 5.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Cash-settled Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Share-based Liabilities Paid	$ 4.0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid / us-gaap_AwardTypeAxis = mwa_CashsettledPerformanceSharesMember
Minimum [Member] | Performance Shares [Member]
Performance Factor			0mwa_PerformanceFactor / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member] | Performance Shares [Member]
Performance Factor			2mwa_PerformanceFactor / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Aggregate carrying value as of the balance sheet date of the liabilities for stock option plans and other equity-based compensation arrangements.

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Per share weighted average intrinsic value of equity-based compensation awards not vested. Excludes stock and unit options.

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