Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability	$ (2.7)	$ (7.2)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	10.9	4.0
Derivative liability	(6.6)	(11.2)
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	10.9	4.0
Derivative liability	(6.6)	(11.2)
Net Amount	4.3	(7.2)
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	3.9	0.0
Derivative liability	(1.3)	0.0
Net Amount	2.6	0.0
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	3.9	0.0
Derivative liability	0.0	0.0
Net Amount	3.9	0.0
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	(1.3)	0.0
Net Amount	(1.3)	0.0
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	7.0	4.0
Derivative liability	(5.3)	(11.2)
Net Amount	1.7	(7.2)
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	4.6	1.0
Derivative liability	(0.1)	(4.1)
Net Amount	4.5	(3.1)
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	0.2	0.0
Derivative liability	(1.2)	(1.2)
Net Amount	(1.0)	(1.2)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 2 | Designated as Hedging Instrument | Purchase | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.4
Derivative liability	0.0	(1.5)
Net Amount	0.0	(1.1)
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	0.2	0.0
Derivative liability	(1.5)	(2.8)
Net Amount	(1.3)	(2.8)
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	2.0	2.6
Derivative liability	(2.5)	(1.6)
Net Amount	$ (0.5)	$ 1.0