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Organization (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Oct. 03, 2019
Jul. 31, 2014
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
Sep. 30, 2015
Sep. 30, 2020
Sep. 30, 2019
Segment Reporting Information [Line Items]                    
Restructuring     $ 2.2   $ 0.6 $ 2.2 $ 0.6   $ 2.8 $ 1.7
Restructuring and other charges     0.8   0.9 2.2 3.3      
Payments           1.6 2.7      
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage 51.00% 49.00%                
Pension costs (benefits) other then service     $ (0.8)   $ (0.8) $ (1.6) $ (1.5)      
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent     21.00%   21.00% 21.00% 21.00%      
Payments to acquire interest in joint venture               $ 1.7    
Proceeds from Other Equity, Gross Amount     $ 4.8     $ 4.8        
Intercompany Loan related to NMTC     12.2     12.2        
Investment Fund Contribution for NMTC     16.5     16.5        
Sub-CDE loan from NMTC     16.2     16.2        
Proceeds from Other Equity       $ 3.9   3.9 $ 0.0      
Restructuring Costs     3.3     14.0        
Severance Costs     0.9     4.8        
Other Restructuring Costs           9.2        
Non-cash restructuring charges           3.6        
Inventory write-down     2.4              
Mueller Co. [Member]                    
Segment Reporting Information [Line Items]                    
Restructuring and other charges     $ (0.7)   $ 0.4 $ (0.6) $ 0.4