Derivative Financial Instruments and Related Hedging Programs, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 5.8	$ 1.6
Derivative Liability	(1.8)	(16.2)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	55.2	72.5
Derivative Asset	5.8	1.6
Derivative Liability	(1.8)	(16.2)
Other Assets, Fair Value Disclosure	286.2	102.5
Fair Value, Measurements, Recurring [Member] | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	231.0	30.0
Fair Value, Measurements, Recurring [Member] | Aluminum Member | Call Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.2
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	37.9	40.3
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Other Assets, Fair Value Disclosure	37.9	40.3
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 1 | Aluminum Member | Call Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		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	17.3	32.2
Derivative Asset	5.8	0.7
Derivative Liability	(1.8)	(15.9)
Other Assets, Fair Value Disclosure	248.3	62.2
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	231.0	30.0
Fair Value, Measurements, Recurring [Member] | Level 2 | Aluminum Member | Call Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.2
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	0.9
Derivative Liability	0.0	(0.3)
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
Fair Value, Measurements, Recurring [Member] | Level 3 | Aluminum Member | Call Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		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	0.9
Derivative Liability	(0.2)	(0.3)
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	3.3	0.3
Derivative Liability	(1.1)	(8.9)
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 Asset		0.2
Derivative Liability	0.0	(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	1.6
Derivative Liability	(0.4)	(6.7)
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	0.0
Derivative Liability	0.0	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	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 Asset		0.0
Derivative Liability	0.0	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
Derivative Liability	0.0	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	0.0
Derivative Liability	(0.2)	0.0
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	3.3	0.3
Derivative Liability	1.1	(8.9)
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 Asset		0.2
Derivative Liability	0.0	(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	1.6
Derivative Liability	(0.4)	(6.7)
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	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 Asset		0.0
Derivative Liability	0.0	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
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Income Approach Valuation Technique | Fair Value, Measurements, Recurring [Member] | Level 3 | Aluminum Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.9
Derivative Liability		(0.3)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		(0.2)
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 Liability	(0.1)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 1 | Foreign Exchange Contract | 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] | Level 1 | Alloy Metal Hedge | 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] | Level 2 | Foreign Exchange Contract | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		(0.2)
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 Liability	0.1
Designated as Hedging Instrument | Fair Value, Measurements, Recurring [Member] | Level 3 | Foreign Exchange Contract | 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] | Level 3 | Alloy Metal Hedge | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ 0.0