Derivatives, Hedging Programs and Other Financial Instruments - Schedule of Fair Value of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Designated as Hedging Instrument | Purchase
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability	$ (14.4)	$ (7.2)
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	2.0	4.0
Derivative liability	(16.4)	(11.2)
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative asset	2.0	4.0
Derivative liability	(16.4)	(11.2)
Net Amount	(14.4)	(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	0.3	1.0
Derivative liability	(6.7)	(4.1)
Net Amount	(6.4)	(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	0.1	0.0
Derivative liability	(2.7)	(1.2)
Net Amount	(2.6)	(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	0.2	0.4
Derivative liability	(1.7)	(1.5)
Net Amount	(1.5)	(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	(2.7)	(2.8)
Net Amount	(2.5)	(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	1.1	2.6
Derivative liability	(2.6)	(1.6)
Net Amount	(1.5)	1.0
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	0.1	0.0
Derivative liability	0.0	0.0
Net Amount	$ 0.1	$ 0.0