Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 14.1	$ 25.3
Derivative liability	(6.7)	(0.8)
Cash and cash equivalents	164.0	51.1
Total	235.9	234.8
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	18.1	23.5
Total	18.1	23.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	14.1	25.3
Derivative liability	(6.7)	(0.8)
Net derivative asset	7.4	24.5
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	(2.2)	(0.1)
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	5.8	0.0
Derivative liability	(4.5)	0.0
Net derivative asset	1.3	0.0
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	8.1	0.0
Derivative liability	(0.1)	0.0
Net derivative asset	8.0	0.0
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	0.0	0.9
Derivative liability	(1.2)	0.0
Net derivative asset		0.9
Net derivative liability	1.2
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	0.1	0.0
Derivative liability	(0.5)	0.0
Net derivative liability	0.4	0.0
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	0.0	0.0
Derivative liability	(0.3)	0.0
Net derivative liability	0.3	0.0
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	0.1	0.0
Derivative liability	(0.1)	0.0
Net derivative asset	0.0	0.0
Not 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	0.0	22.5
Derivative liability	0.0	0.0
Net derivative asset	0.0	22.5
Not 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	0.0	1.7
Derivative liability	0.0	(0.1)
Net derivative asset	0.0	1.6
Not 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	0.0	0.2
Derivative liability	0.0	(0.5)
Net derivative liability	0.0	0.3
Not 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	0.0	0.0
Derivative liability	0.0	(0.1)
Net derivative liability	0.0	0.1
Not 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	0.0
Derivative liability	0.0	(0.1)
Net derivative liability	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	71.9	183.7
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	145.9	27.6
Short-term investments	71.9	183.7
Total	217.8	211.3
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