Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 135.0	$ 51.1
Derivative Asset	(11.4)	(25.3)
Derivative Liability	(5.5)	(0.8)
Other Assets, Fair Value Disclosure	218.8	234.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	17.3	23.5
Other Assets, Fair Value Disclosure	17.3	23.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	117.7	27.6
Derivative Asset	11.4	25.3
Derivative Liability	(5.5)	(0.8)
Derivative Asset, Fair Value, Gross Asset	(5.9)	(24.5)
Other Assets, Fair Value Disclosure	201.5	211.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Liability	0.0	(0.1)
Not Designated as Hedging Instrument | Purchase | Midwest premium swap contracts | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Asset	0.0	1.6
Not Designated as Hedging Instrument | Purchase | Natural Gas Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Liability	0.0	(0.3)
Not Designated as Hedging Instrument | Purchase | Electricity Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Liability	0.0	(0.1)
Not Designated as Hedging Instrument | Sales | Aluminum Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Asset	0.0	22.5
Designated as Hedging Instrument | Purchase
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(1.6)	(0.1)
Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.2)	0.0
Derivative Liability	0.0	0.0
Derivative Asset, Fair Value, Gross Asset	0.2	0.0
Designated as Hedging Instrument | Purchase | Midwest premium swap contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(6.2)	0.0
Derivative Liability	0.1	0.0
Derivative Asset, Fair Value, Gross Asset	6.1	0.0
Designated as Hedging Instrument | Purchase | Natural Gas Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Liability	(0.4)	0.0
Designated as Hedging Instrument | Purchase | Electricity Member | Fair Value, Measurements, Recurring [Member] | 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
Derivative Asset, Fair Value, Gross Liability	(0.3)	0.0
Designated as Hedging Instrument | Purchase | Alloy Metal Hedge | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.3)	(0.9)
Derivative Liability	0.2	0.0
Derivative Asset, Fair Value, Gross Asset	0.1	0.9
Designated as Hedging Instrument | Sales | Aluminum Member | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(4.6)	0.0
Derivative Liability	4.4	0.0
Derivative Asset, Fair Value, Gross Asset	0.2	0.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	83.8	183.7
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	83.8	183.7
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(9.0)	(18.9)
Other Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(2.4)	(6.4)
Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(3.1)	(0.3)
Other Noncurrent Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ (2.4)	$ (0.5)