Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability	$ 0.0	$ (2.7)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	48.2	10.9
Derivative liability	(1.9)	(6.6)
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	48.2	10.9
Derivative liability	(1.9)	(6.6)
Net Amount	46.3	4.3
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	2.0	3.9
Derivative liability	(1.2)	(1.3)
Net Amount	0.8	2.6
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	2.0	3.9
Derivative liability	(0.5)	0.0
Net Amount	1.5	3.9
Fair Value, Measurements, Recurring | Level 2 | Not Designated as Hedging Instrument | Sales | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.0
Derivative liability	(0.7)	(1.3)
Net Amount	(0.7)	(1.3)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	46.2	7.0
Derivative liability	(0.7)	(5.3)
Net Amount	45.5	1.7
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	23.9	4.6
Derivative liability	(0.2)	(0.1)
Net Amount	23.7	4.5
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	7.1	0.2
Derivative liability	(0.1)	(1.2)
Net Amount	7.0	(1.0)
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	5.8	0.2
Derivative liability	0.0	(1.5)
Net Amount	5.8	(1.3)
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	9.4	2.0
Derivative liability	(0.2)	(2.5)
Net Amount	9.2	(0.5)
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	(0.2)	0.0
Net Amount	$ (0.2)	$ 0.0