Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 264.3	$ 125.6
Derivative Asset	(4.0)	(4.3)
Derivative Liability	(11.2)	(15.9)
Other Assets, Fair Value Disclosure	343.0	162.3
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	78.7	36.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	28.2	22.9
Other Assets, Fair Value Disclosure	28.2	22.9
Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	236.1	102.7
Derivative Asset	(4.0)	(4.3)
Derivative Liability	(11.2)	(15.9)
Other Assets, Fair Value Disclosure	314.8	139.4
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	78.7	36.7
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Designated as Hedging Instrument | Level 2 | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(1.0)	(0.1)
Derivative Liability	(4.1)	(13.2)
Designated as Hedging Instrument | Level 2 | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(0.1)
Derivative Liability	0.0	0.0
Designated as Hedging Instrument | Level 2 | Midwest premium swap contracts | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(3.2)
Derivative Liability	(1.2)	(0.5)
Designated as Hedging Instrument | Level 2 | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(0.2)
Derivative Liability	(2.8)	(0.5)
Designated as Hedging Instrument | Level 2 | Electricity [Member] | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(2.6)	(0.7)
Derivative Liability	(1.6)	0.0
Designated as Hedging Instrument | Level 2 | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.4)	0.0
Derivative Liability	$ (1.5)	$ (1.7)