Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 125.6	$ 51.1
Derivative Asset	(4.3)	(25.3)
Derivative Liability	(15.9)	(0.8)
Other Assets, Fair Value Disclosure	162.3	234.8
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	36.7	183.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	22.9	23.5
Other Assets, Fair Value Disclosure	22.9	23.5
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
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	102.7	27.6
Derivative Asset	(4.3)	(25.3)
Derivative Liability	(15.9)	(0.8)
Other Assets, Fair Value Disclosure	139.4	211.3
Fair Value, Inputs, Level 2 [Member] | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	36.7	183.7
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
Not Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(22.5)
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	(0.1)
Not Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Midwest premium swap contracts | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(1.7)
Derivative Liability	0.0	(0.1)
Not Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | 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	0.0	(0.5)
Not Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Electricity [Member] | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	(0.1)
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.1)	0.0
Derivative Liability	(13.2)	0.0
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Aluminum Member | Short
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
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Midwest premium swap contracts | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(3.2)	0.0
Derivative Liability	(0.5)	0.0
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.2)	0.0
Derivative Liability	(0.5)	0.0
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Electricity [Member] | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.7)	0.0
Derivative Liability	0.0	0.0
Designated as Hedging Instrument | Fair Value, Inputs, Level 2 [Member] | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	(0.9)
Derivative Liability	$ (1.7)	$ 0.0