Segment Information (Schedule Of Selected Supplemental Balance Sheet Information) (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
Segment Reporting Information [Line Items]
Intercompany sales	$ 265.3	$ 228.5	$ 257.7	$ 212.6	$ 266.9	$ 274.3	$ 234.0	$ 192.8	$ 964.1	$ 968.0	$ 916.0
Operating income	40.7	$ 20.0	$ 35.8	$ 20.3	39.0	$ 47.2	$ 22.2	$ 15.9	116.8	124.3	121.7
Depreciation and amortization									57.8	53.0	43.7
2016									13.0	16.3	10.5
Capital expenditures									(67.7)	(86.6)	(55.7)
Property, plant and equipment, net	253.8				217.1				253.8	217.1
Total assets	1,395.0				1,337.3				$ 1,395.0	1,337.3
Document Period End Date									Sep. 30, 2020
Intangible Assets, Net (Including Goodwill)	508.7				529.4				$ 508.7	529.4
Mueller Co. [Member]
Segment Reporting Information [Line Items]
Intercompany sales									885.5	871.0	818.8
Operating income									186.7	182.3	180.1
Depreciation and amortization									49.1	44.8	37.4
2016									0.6	1.7	0.1
Capital expenditures									(64.5)	(80.4)	(47.3)
Total assets	1,149.8				1,107.8				1,149.8	1,107.8
Intangible Assets, Net (Including Goodwill)	490.8				508.2				490.8	508.2
Mueller Technologies [Member]
Segment Reporting Information [Line Items]
Intercompany sales									78.6	97.0	97.2
Operating income									(13.1)	(8.7)	(24.4)
Depreciation and amortization									8.5	7.9	6.1
2016									0.1	0.0	0.1
Capital expenditures									(2.8)	(5.5)	(8.3)
Total assets	93.2				100.3				93.2	100.3
Intangible Assets, Net (Including Goodwill)	17.9				21.2				17.9	21.2
Corporate [Member]
Segment Reporting Information [Line Items]
Intercompany sales									0.0	0.0	0.0
Operating income									(56.8)	(49.3)	(34.0)
Depreciation and amortization									0.2	0.3	0.2
2016									12.3	14.6	10.3
Capital expenditures									(0.4)	(0.7)	$ (0.1)
Total assets	152.0				129.2				152.0	129.2
Intangible Assets, Net (Including Goodwill)	$ 0.0				$ 0.0				$ 0.0	$ 0.0