Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 1.8	$ 4.0
Derivative liability	(30.8)	(11.2)
Cash and cash equivalents	307.2	264.3
Total	346.5	343.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	199.6	28.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	1.8	4.0
Derivative liability	(30.8)	(11.2)
Derivative Assets (Liabilities), at Fair Value, Net	(29.0)	(7.2)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Total	0.0	0.0
Designated as Hedging Instrument | Purchase
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	(29.2)	(7.2)
Debt Securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	39.3	78.7
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0.0	0.0
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	146.9	314.8
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0.0	0.0
Cash Flow Hedges | Designated as Hedging Instrument | Purchase | Aluminum | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0.0	1.0
Derivative liability	(14.2)	(4.1)
Derivative Assets (Liabilities), at Fair Value, Net	(14.2)	(3.1)
Cash Flow Hedges | Designated as Hedging Instrument | Purchase | Midwest premium swap contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.0
Derivative liability	(4.3)	(1.2)
Derivative Assets (Liabilities), at Fair Value, Net	(4.3)	(1.2)
Cash Flow Hedges | Designated as Hedging Instrument | Purchase | Alloy Metal Hedge | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.4
Derivative liability	(5.5)	(1.5)
Derivative Assets (Liabilities), at Fair Value, Net	(5.5)	(1.1)
Cash Flow Hedges | Designated as Hedging Instrument | Purchase | Natural Gas | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0.1	0.0
Derivative liability	(3.0)	(2.8)
Derivative Assets (Liabilities), at Fair Value, Net	(2.9)	(2.8)
Cash Flow Hedges | Designated as Hedging Instrument | Purchase | Electricity | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	1.6	2.6
Derivative liability	(2.8)	(1.6)
Derivative Assets (Liabilities), at Fair Value, Net	(1.2)	1.0
Cash Flow Hedges | Designated as Hedging Instrument | Sales | Aluminum | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
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
Derivative Assets (Liabilities), at Fair Value, Net	0.1	0.0
Fair Value Hedges | Designated as Hedging Instrument | Purchase | Electricity | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0.0	0.0
Derivative liability	(1.0)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	$ (1.0)	$ 0.0