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Commitments and Contingencies (Details)
In Millions, unless otherwise specified
 6 Months Ended
Mar. 31, 2015
USD ($)
Mar. 31, 2015
CAD
Mar. 31, 2014
USD ($)
Dec. 31, 2014
Prior To 1995 [member]
USD ($)
Mar. 31, 2015
From 2000 to 2005 [Member]
USD ($)
Loss Contingency, Damages Sought, Value   10.0us-gaap_LossContingencyDamagesSoughtValue      
Walter Energy tax assessment 34.0mwa_AmountOfTaxClaimedForIssuesCurrentlyInDispute     33.0mwa_AmountOfTaxClaimedForIssuesCurrentlyInDispute
/ mwa_CommitmentsAndContingenciesTimeFrameAxis
= mwa_PriorTo1995Member		 82.2mwa_AmountOfTaxClaimedForIssuesCurrentlyInDispute
/ mwa_CommitmentsAndContingenciesTimeFrameAxis
= mwa_From2000To2005Member
Walter Energy tax assessment prior to 1995 related to timing 21.0mwa_AmountOfTaxClaimedForIssuesAtFormerAffiliateRelatingToTimingOfDeductionsOnly        
Amount payable pending completion of an IRS audit 11.6mwa_AmountReceivablePendingCompletionOfAnIRSAudit        
Proceeds from Sale of Productive Assets $ 4.8us-gaap_ProceedsFromSaleOfProductiveAssets   $ 0.3us-gaap_ProceedsFromSaleOfProductiveAssets