Derivatives, Hedging Programs and Other Financial Instruments, Material Derivative Positions (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Asset	$ 4.0	$ 4.3
Derivative Liability	(11.2)	(15.9)
Level 2
Derivative [Line Items]
Derivative Asset	4.0	4.3
Derivative Liability	(11.2)	(15.9)
Derivative Assets (Liabilities), at Fair Value, Net	(7.2)	(11.6)
Level 2 | Designated as Hedging Instrument | Aluminum | Long
Derivative [Line Items]
Derivative Asset	1.0	0.1
Derivative Liability	(4.1)	(13.2)
Derivative Assets (Liabilities), at Fair Value, Net	(3.1)	(13.1)
Level 2 | Designated as Hedging Instrument | Aluminum | Short
Derivative [Line Items]
Derivative Asset	0.0	0.1
Derivative Liability	0.0	0.0
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.1
Level 2 | Designated as Hedging Instrument | Midwest premium swap contracts | Long
Derivative [Line Items]
Derivative Asset	0.0	3.2
Derivative Liability	(1.2)	(0.5)
Derivative Assets (Liabilities), at Fair Value, Net	(1.2)	2.7
Level 2 | Designated as Hedging Instrument | Alloy Metal Hedge | Long
Derivative [Line Items]
Derivative Asset	0.4	0.0
Derivative Liability	(1.5)	(1.7)
Derivative Assets (Liabilities), at Fair Value, Net	(1.1)	(1.7)
Level 2 | Designated as Hedging Instrument | Natural Gas Member | Long
Derivative [Line Items]
Derivative Asset	0.0	0.2
Derivative Liability	(2.8)	(0.5)
Derivative Assets (Liabilities), at Fair Value, Net	(2.8)	(0.3)
Level 2 | Designated as Hedging Instrument | Electricity [Member] | Long
Derivative [Line Items]
Derivative Asset	2.6	0.7
Derivative Liability	(1.6)	0.0
Derivative Assets (Liabilities), at Fair Value, Net	$ 1.0	$ 0.7