Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability	$ (0.2)	$ (2.7)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	20.1	10.9
Derivative liability	(4.3)	(6.6)
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	20.1	10.9
Derivative liability	(4.3)	(6.6)
Net Amount	15.8	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	4.2	3.9
Derivative liability	(1.3)	(1.3)
Net Amount	2.9	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	4.2	3.9
Derivative liability	(0.1)	0.0
Net Amount	4.1	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	(1.2)	(1.3)
Net Amount	(1.2)	(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	15.9	7.0
Derivative liability	(3.0)	(5.3)
Net Amount	12.9	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	8.9	4.6
Derivative liability	(0.1)	(0.1)
Net Amount	8.8	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	3.0	0.2
Derivative liability	0.0	(1.2)
Net Amount	3.0	(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	0.3	0.2
Derivative liability	(1.1)	(1.5)
Net Amount	(0.8)	(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	3.7	2.0
Derivative liability	(1.7)	(2.5)
Net Amount	2.0	(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	(0.1)	0.0
Net Amount	$ (0.1)	$ 0.0