Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability	$ 0.0	$ 0.0
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	50.0	26.7
Derivative liability	(1.9)	(2.0)
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	50.0	26.7
Derivative liability	(1.9)	(2.0)
Net Amount	48.1	24.7
Fair Value, Measurements, Recurring | Level 2 | Not Designated as Hedging Instrument | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	2.5	1.6
Derivative liability	0.0	(0.1)
Net Amount	2.5	1.5
Fair Value, Measurements, Recurring | Level 2 | Not Designated as Hedging Instrument | Purchase | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	2.5	1.6
Derivative liability	0.0	(0.1)
Net Amount	2.5	1.5
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	47.5	25.1
Derivative liability	(1.9)	(1.9)
Net Amount	45.6	23.2
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Purchase | Aluminum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	26.5	12.2
Derivative liability	(1.0)	(1.1)
Net Amount	25.5	11.1
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Purchase | Midwest premium swap contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	6.9	5.5
Derivative liability	(0.2)	(0.1)
Net Amount	6.7	5.4
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Purchase | Natural Gas
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	8.0	3.0
Derivative liability	0.0	(0.1)
Net Amount	8.0	2.9
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Purchase | Electricity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	6.1	4.4
Derivative liability	0.0	(0.6)
Net Amount	6.1	3.8
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Sales | Aluminum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.0
Derivative liability	(0.7)	0.0
Net Amount	$ (0.7)	$ 0.0