Segment Information (Revenue and Long-Lived Assets by Geographical Areas) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 265.3	$ 228.5	$ 257.7	$ 212.6	$ 266.9	$ 274.3	$ 234.0	$ 192.8	$ 964.1	$ 968.0	$ 916.0
Intangible Assets, Net (Including Goodwill)	508.7				529.4				508.7	529.4
Total assets	1,395.0				1,337.3				$ 1,395.0	1,337.3
Document Period End Date									Sep. 30, 2020
Property, plant and equipment, net	253.8				217.1				$ 253.8	217.1
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Property, plant and equipment, net	234.7				201.3				234.7	201.3
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Property, plant and equipment, net	12.8				9.7				12.8	9.7
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Property, plant and equipment, net	6.3				6.1				6.3	6.1
Mueller Co. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									885.5	871.0	818.8
Intangible Assets, Net (Including Goodwill)	490.8				508.2				490.8	508.2
Total assets	1,149.8				1,107.8				1,149.8	1,107.8
Mueller Co. [Member] | Central Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									222.2	214.2
Mueller Co. [Member] | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									788.9	772.8
Mueller Co. [Member] | Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									65.5	69.0
Mueller Co. [Member] | Northeast Region [Member] [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									187.5	183.1
Mueller Co. [Member] | Southeast Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									162.3	162.7
Mueller Co. [Member] | Western Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									216.9	212.8
Mueller Co. [Member] | Other International Locations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									31.1	29.2
Mueller Technologies [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									78.6	97.0	97.2
Intangible Assets, Net (Including Goodwill)	17.9				21.2				17.9	21.2
Total assets	93.2				100.3				93.2	100.3
Mueller Technologies [Member] | Central Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									18.7	27.8
Mueller Technologies [Member] | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									74.1	92.0
Mueller Technologies [Member] | Northeast Region [Member] [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									19.7	20.4
Mueller Technologies [Member] | Southeast Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									22.1	33.5
Mueller Technologies [Member] | Western Region [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									13.6	10.3
Mueller Technologies [Member] | Other International Locations [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									4.5	5.0
Customer Concentration Risk [Member] | Accounts Receivable [Member] | Ferguson Enterprises
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts and Other Receivables, Net, Current	26.1				$ 25.8				26.1	$ 25.8
Customer Concentration Risk [Member] | Accounts Receivable [Member] | HDS IP Holdings, LLC [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts and Other Receivables, Net, Current	$ 37.1								$ 37.1		$ 31.9
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | Major Customers, Group One
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									53.00%	53.00%	54.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | Major Customers, Group Two
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									34.00%	34.00%	34.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | Ferguson Enterprises
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									17.00%	18.00%	19.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | Ferguson Enterprises | Mueller Co. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									16.00%	17.00%	18.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | Ferguson Enterprises | Mueller Technologies [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									22.00%	30.00%	28.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | HDS IP Holdings, LLC [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									17.00%	16.00%	15.00%
Customer Concentration Risk [Member] | Sales Revenue, Goods, Gross [Member] | HDS IP Holdings, LLC [Member] | Mueller Co. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
10 largest customers									19.00%	18.00%	17.00%