Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 18.7	$ (3.4)	$ (6.8)
Foreign Exchange Contract | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	0.0	(0.3)	0.0
Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(0.1)	0.0	0.0
Operational Risk Hedges | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	18.7	(3.4)	(6.8)
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(2.0)	(27.3)	6.9
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(5.0)	(5.4)	1.0
Cost of Sales | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	0.0	(1.9)	(0.1)
Cost of Sales | Operational Risk Hedges | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(7.1)	(34.6)	7.8
Other Operating Income (Expense) | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(0.1)	0.0	0.0
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	10.8	(4.6)	(2.6)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	7.9	(0.5)	(6.0)
Gain (Loss) on Derivative Instruments | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	0.0	1.7	(1.8)
Gain (Loss) on Derivative Instruments | Operational Risk Hedges | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	18.7	(3.4)	(10.4)
Other Expense | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	0.0	0.0	3.6
Other Expense | Option Assets relating to the Convertible Notes3 | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	0.0	0.0	5.2
Other Expense | Bifurcated Conversion Feature of the Convertible Notes | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 0.0	$ 0.0	$ (1.6)