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Segment Information (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 USD ($) business_segments customer | Sep. 30, 2016 USD ($) | Sep. 30, 2015 | Sep. 30, 2014
Segment Reporting [Abstract] |  |  |  |
Number of Reportable Segments | business_segments | 2 |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Receivables, net | $ 145.3 | $ 131.8 |  |
Intangible Assets, Net (Including Goodwill) | 439.3 | 434.6 |  |
Total assets | $ 1,258.3 | 1,280.6 |  |
Document Period End Date | Sep. 30, 2017 |  |  |
Mueller Co. [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Intangible Assets, Net (Including Goodwill) | $ 417.2 | 416.9 |  |
Total assets | 792.6 | 750.4 |  |
Mueller Technologies [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Intangible Assets, Net (Including Goodwill) | 22.1 | 17.7 |  |
Total assets | $ 88.4 | $ 86.1 |  |
Customer Concentration Risk [Member] | Major Customers, Group One |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Number of major customers | customer | 10 |  |  |
Customer Concentration Risk [Member] | Major Customers, Group One | Sales Revenue, Goods, Gross [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 53.00% |  |  |
Customer Concentration Risk [Member] | Major Customers, Group Two |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Number of major customers | customer | 2 |  |  |
Customer Concentration Risk [Member] | Major Customers, Group Two | Sales Revenue, Goods, Gross [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 34.00% |  |  |
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 18.00% | 17.00% | 15.00% |
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Co. [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 17.00% |  |  |
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Technologies [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 24.00% |  |  |
Customer Concentration Risk [Member] | Ferguson Enterprises | Accounts Receivable [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Receivables, net | $ 20.0 | $ 21.9 |  |
Customer Concentration Risk [Member] | HD Supply | Sales Revenue, Goods, Gross [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 16.00% | 16.00% |  | 16.00%
Customer Concentration Risk [Member] | HD Supply | Sales Revenue, Goods, Gross [Member] | Mueller Co. [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Concentration risk, percentage of benchmark | 17.00% |  |  |
Customer Concentration Risk [Member] | HD Supply | Accounts Receivable [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Receivables, net | $ 27.3 | $ 23.8 |  |

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