Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)			$ 16.9	$ (7.7)
Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	$ 2.0	$ (1.7)	16.9	(7.7)
Foreign Currency | Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.1	0.0	0.2	(0.2)
Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(0.4)	(9.0)	(4.1)	(18.8)
Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(1.3)	(10.5)	(8.3)	(24.0)
Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(0.9)	(1.2)	(4.2)	(3.8)
Electricity | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	0.0	(0.3)	0.0	(1.4)
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	1.7	(0.3)	11.7	(8.8)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.3	(1.0)	5.2	(0.2)
Gain (Loss) on Derivative Instruments | Electricity | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.0	(0.4)	0.0	1.3
Other Expense | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.0	0.0	0.0	0.0
Other Expense | Call Option | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.0	0.0	0.0	10.2
Other Expense | Embedded Derivative Financial Instruments [Member] | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	$ 0.0	$ 0.0	$ 0.0	$ (10.2)