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Segment Information (Narrative) (Details)
$ in Millions
12 Months Ended
Sep. 30, 2020
USD ($)
business_segments
Sep. 30, 2019
USD ($)
Sep. 30, 2018
Segment Reporting [Abstract]      
Number of Reportable Segments | business_segments 2    
Revenue, Major Customer [Line Items]      
Receivables, net $ 180.8 $ 172.8  
Intangible Assets, Net (Including Goodwill) 508.7 529.4  
Total assets $ 1,395.0 1,337.3  
Document Period End Date Sep. 30, 2020    
Mueller Co. [Member]      
Revenue, Major Customer [Line Items]      
Intangible Assets, Net (Including Goodwill) $ 490.8 508.2  
Total assets 1,149.8 1,107.8  
Mueller Technologies [Member]      
Revenue, Major Customer [Line Items]      
Intangible Assets, Net (Including Goodwill) 17.9 21.2  
Total assets $ 93.2 $ 100.3  
Customer Concentration Risk [Member] | Major Customers, Group One | Sales Revenue, Goods, Gross [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 53.00% 53.00% 54.00%
Customer Concentration Risk [Member] | Major Customers, Group Two | Sales Revenue, Goods, Gross [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 34.00% 34.00% 34.00%
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 17.00% 18.00% 19.00%
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Co. [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 16.00% 17.00% 18.00%
Customer Concentration Risk [Member] | Ferguson Enterprises | Sales Revenue, Goods, Gross [Member] | Mueller Technologies [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 22.00% 30.00% 28.00%
Customer Concentration Risk [Member] | HDS IP Holdings, LLC [Member] | Sales Revenue, Goods, Gross [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 17.00% 16.00% 15.00%
Customer Concentration Risk [Member] | HDS IP Holdings, LLC [Member] | Sales Revenue, Goods, Gross [Member] | Mueller Co. [Member]      
Revenue, Major Customer [Line Items]      
10 largest customers 19.00% 18.00% 17.00%