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Segment Information (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2018 USD ($) business_segments customer | Sep. 30, 2017 USD ($) | Sep. 30, 2016
Segment Reporting [Abstract] |  |  |
Number of Reportable Segments | business_segments | 2 |  |
Revenue, Major Customer [Line Items] |  |  |
Receivables, net | $ 164.3 | $ 145.3 |
Intangible Assets, Net (Including Goodwill) | 420.2 | 439.3 |
Total assets | $ 1,291.9 | 1,258.3 |
Document Period End Date | Sep. 30, 2018 |  |
Mueller Co. [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Intangible Assets, Net (Including Goodwill) | $ 396.9 | 417.2 |
Total assets | 843.9 | 792.6 |
Mueller Technologies [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Intangible Assets, Net (Including Goodwill) | 23.3 | 22.1 |
Total assets | $ 87.1 | $ 88.4 |
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, Net [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Concentration risk, percentage of benchmark | 54.00% |  |
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 | 19.00% | 18.00% | 17.00%
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Co. [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Concentration risk, percentage of benchmark | 18.00% |  |
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Technologies [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Concentration risk, percentage of benchmark | 28.00% |  |
Customer Concentration Risk [Member] | Ferguson Enterprises | Accounts Receivable [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Receivables, net | $ 29.7 | $ 20.0 |
Customer Concentration Risk [Member] | HD Supply | Sales Revenue, Goods, Gross [Member] |  |  |
Revenue, Major Customer [Line Items] |  |  |
Concentration risk, percentage of benchmark | 15.00% | 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 | $ 31.7 | $ 27.3 |

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