Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 6.7	$ 4.3
Derivative liability	(9.2)	(15.9)
Cash and cash equivalents	115.3	125.6
Total	137.6	162.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	29.3	22.9
Total	29.3	22.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	6.7	4.3
Derivative liability	(9.2)	(15.9)
Derivative Assets (Liabilities), at Fair Value, Net	(2.5)	(11.6)
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	(6.0)	(12.6)
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	0.6	0.1
Derivative liability	(7.9)	(13.2)
Derivative Assets (Liabilities), at Fair Value, Net	(7.3)	(13.1)
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	2.8	3.2
Derivative liability	(0.2)	(0.5)
Derivative Assets (Liabilities), at Fair Value, Net	2.6	2.7
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	0.9	0.0
Derivative liability	(0.3)	(1.7)
Derivative Assets (Liabilities), at Fair Value, Net	0.6	(1.7)
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	0.4	0.2
Derivative liability	(0.8)	(0.5)
Derivative Assets (Liabilities), at Fair Value, Net	(0.4)	(0.3)
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	2.0	0.7
Derivative liability	0.0	0.0
Derivative Assets (Liabilities), at Fair Value, Net	2.0	0.7
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	0.0	0.1
Derivative liability	0.0	0.0
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.1
Debt Securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	22.3	36.7
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]
Cash and cash equivalents	86.0	102.7
Short-term investments	22.3	36.7
Total	108.3	139.4
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