Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 0.0	$ (2.7)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	26.7	10.9
Liabilities	(2.0)	(6.6)
Fair Value, Measurements, Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	26.7	10.9
Liabilities	(2.0)	(6.6)
Net Amount	24.7	4.3
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	1.6	3.9
Liabilities	(0.1)	(1.3)
Net Amount	1.5	2.6
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument | Purchase | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	1.6	3.9
Liabilities	(0.1)	0.0
Net Amount	1.5	3.9
Fair Value, Measurements, Recurring | Level 1 | Not Designated as Hedging Instrument | Sales | Alloying Metals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Liabilities	0.0	(1.3)
Net Amount	0.0	(1.3)
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	25.1	7.0
Liabilities	(1.9)	(5.3)
Net Amount	23.2	1.7
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	12.2	4.6
Liabilities	(1.1)	(0.1)
Net Amount	11.1	4.5
Fair Value, Measurements, Recurring | Cash Flow Hedges | Level 1 | Designated as Hedging Instrument | Purchase | Midwest premium swap contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	5.5	0.2
Liabilities	(0.1)	(1.2)
Net Amount	5.4	(1.0)
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	3.0	0.2
Liabilities	(0.1)	(1.5)
Net Amount	2.9	(1.3)
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	4.4	2.0
Liabilities	(0.6)	(2.5)
Net Amount	$ 3.8	$ (0.5)