Derivative Financial Instruments and Related Hedging Programs, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ (25.3)	$ (5.8)
Derivative Liability	(0.8)	(1.8)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	51.1	55.2
Derivative Asset	(25.3)	(5.8)
Derivative Liability	(0.8)	(1.8)
Other Assets, Fair Value Disclosure	234.8	286.2
Fair Value, Measurements, Recurring [Member] | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	183.7	231.0
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	23.5	37.9
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Other Assets, Fair Value Disclosure	23.5	37.9
Fair Value, Measurements, Recurring [Member] | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	27.6	17.3
Derivative Asset	(25.3)	(5.8)
Derivative Liability	(0.8)	(1.8)
Other Assets, Fair Value Disclosure	211.3	248.3
Fair Value, Measurements, Recurring [Member] | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	183.7	231.0
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Aluminum Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.9
Derivative Liability		(0.2)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(22.5)	(3.3)
Derivative Liability		(1.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Midwest premium swap contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(1.7)
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.2)	(1.6)
Derivative Liability	(0.5)	(0.4)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Electricity Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Aluminum Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.0
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Midwest premium swap contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Electricity Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Aluminum Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.9
Derivative Liability		0.2
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(22.5)	(3.3)
Derivative Liability		(1.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Midwest premium swap contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(1.7)
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.2)	(1.6)
Derivative Liability	(0.5)	(0.4)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Electricity Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Aluminum Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.0
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Aluminum Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Aluminum Member | Short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Midwest premium swap contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Natural Gas Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Electricity Member | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.9)
Derivative Liability	0.0	(0.1)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0	0.0
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 2 | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(0.9)
Derivative Liability	0.0	(0.1)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0	0.0
Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.3)	(0.8)
Other Noncurrent Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	(0.5)	(1.0)
Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	(18.9)	(5.0)
Other Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ (6.4)	$ (0.8)