Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ (1.0)	$ (3.3)
Not Designated as Hedging Instrument
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net Amount	0.0	0.0
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	5.9	5.9
Liabilities	(2.9)	(5.3)
Fair Value, Measurements, Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	5.9	5.9
Liabilities	(2.9)	(5.3)
Net Amount	3.0	0.6
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	4.7
Liabilities	(0.9)
Net Amount	(0.6)	4.7
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	3.4	0.7
Liabilities	(0.6)	(3.9)
Net Amount	2.8	(3.2)
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	0.4	0.5
Liabilities	(0.3)	(1.4)
Net Amount	0.1	$ (0.9)
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	0.5
Net Amount	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	0.7
Liabilities	(0.1)
Net Amount	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)