Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 19.4	$ 18.7	$ (3.4)
Operational Risk Hedges | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	(19.4)	(18.7)	3.4
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	20.4	(2.0)	(27.3)
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(0.7)	(5.0)	(5.4)
Cost of Sales | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	(0.9)	0.0	0.0
Cost of Sales | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	0.0	0.0	(1.9)
Cost of Sales | Operational Risk Hedges | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	20.7	(7.1)	(34.6)
Other Operating Income (Expense) | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains	0.1	(0.1)	0.0
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	(20.9)	(10.8)	4.6
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	1.4	(7.9)	0.5
Gain (Loss) on Derivative Instruments | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 0.1	$ 0.0	$ (1.7)