Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ (2.0)	$ (1.0)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	8.1	5.9
Liabilities	(3.2)	(2.9)
Fair Value, Measurements, Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	8.1	5.9
Liabilities	(3.2)	(2.9)
Net Amount	4.9	3.0
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	(2.2)
Net Amount	(2.2)
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument | Purchase | Electricity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	(1.9)
Net Amount	(1.9)
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument | Sales | Electricity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	(0.3)
Net Amount	(0.3)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	8.1	5.9
Liabilities	(1.0)	(2.9)
Net Amount	7.1	3.0
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Natural Gas
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	0.3	0.3
Liabilities	(0.7)	(0.9)
Net Amount	(0.4)	(0.6)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Aluminum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	4.3	3.4
Liabilities	(0.2)	(0.6)
Net Amount	4.1	2.8
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Fixed Price Purchase Contracts for MWTP
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	0.3	0.4
Liabilities	(0.1)	(0.3)
Net Amount	0.2	0.1
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Fixed Price Sale Contracts for MWTP
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		0.1
Liabilities		(0.2)
Net Amount		(0.1)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Foreign Currency
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	0.2	0.5
Net Amount	0.2	0.5
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	3.0	0.7
Liabilities		(0.1)
Net Amount	$ 3.0	0.6
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Electricity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		0.5
Liabilities		(0.6)
Net Amount		(0.1)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Sales | Aluminum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities		(0.2)
Net Amount		$ (0.2)