Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total Other items, net	$ (0.8)	$ 0.0
Interest expense	(47.3)	(46.8)
Accumulated other comprehensive loss	(305.3)		$ (307.7)
Loss / (earnings) before income taxes	87.3	58.7
Income tax (benefit)/expense	(20.2)	(12.9)
Net earnings	67.1	46.1
Zinc contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss	7.4		3.4
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net earnings	5.3	0.5
Gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss	(8.0)		(4.1)
Gains and losses on cash flow hedges | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total Other items, net	2.6	(1.9)
Interest expense	(1.3)	0.5
Loss / (earnings) before income taxes	4.7	(1.1)
Income tax (benefit)/expense	1.1	0.2
Net earnings	3.6	(0.9)
Gains and losses on cash flow hedges | Amount Reclassified from AOCI | Zinc contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss	0.8	0.3
Amortization of defined benefit pension items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss	(164.0)		$ (163.5)
Amortization of defined benefit pension items | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Actuarial loss	(2.2)	1.9
Income tax (benefit)/expense	0.5	(0.5)
Net earnings	$ 1.7	$ 1.4