Derivatives, Hedging Programs and Other Financial Instruments, Material Derivative Positions (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Asset	$ 4.3	$ 25.3
Derivative Liability	(15.9)	(0.8)
Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative Asset	4.3	25.3
Derivative Liability	(15.9)	(0.8)
Derivative Assets (Liabilities), at Fair Value, Net	(11.6)	24.5
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument | Aluminum | Long
Derivative [Line Items]
Derivative Asset	0.0	22.5
Derivative Liability	0.0	0.0
Derivative Assets (Liabilities), at Fair Value, Net	0.0	22.5
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument | Aluminum | Short
Derivative [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	(0.1)
Derivative Assets (Liabilities), at Fair Value, Net	0.0	(0.1)
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument | Midwest premium swap contracts | Long
Derivative [Line Items]
Derivative Asset	0.0	1.7
Derivative Liability	0.0	(0.1)
Derivative Assets (Liabilities), at Fair Value, Net	0.0	1.6
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument | Natural Gas Member | Long
Derivative [Line Items]
Derivative Asset	0.0	0.2
Derivative Liability	0.0	(0.5)
Derivative Assets (Liabilities), at Fair Value, Net	0.0	(0.3)
Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument | Electricity [Member] | Long
Derivative [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	(0.1)
Derivative Assets (Liabilities), at Fair Value, Net	0.0	(0.1)
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Aluminum | Long
Derivative [Line Items]
Derivative Asset	0.1	0.0
Derivative Liability	(13.2)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	(13.1)	0.0
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Aluminum | Short
Derivative [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, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Midwest premium swap contracts | Long
Derivative [Line Items]
Derivative Asset	3.2	0.0
Derivative Liability	(0.5)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	2.7	0.0
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Alloy Metal Hedge | Long
Derivative [Line Items]
Derivative Asset	0.0	0.9
Derivative Liability	(1.7)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	(1.7)	0.9
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Natural Gas Member | Long
Derivative [Line Items]
Derivative Asset	0.2	0.0
Derivative Liability	(0.5)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	(0.3)	0.0
Fair Value, Inputs, Level 2 [Member] | Designated as Hedging Instrument | Electricity [Member] | Long
Derivative [Line Items]
Derivative Asset	0.7	0.0
Derivative Liability	0.0	0.0
Derivative Assets (Liabilities), at Fair Value, Net	$ 0.7	$ 0.0