Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)			$ 14.9	$ (6.0)
Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	$ 10.9	$ (1.5)	14.9	(6.0)
Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(2.6)	(8.8)	(7.0)	(13.5)
Unrealized gains (losses)	10.9	(1.5)	14.9	(6.0)
Foreign Currency Member | Operational Risk Hedges | Designated as Hedging Instrument [Member]
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	(0.1)	(0.2)	0.1	(0.2)
Aluminum | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(1.0)	(7.1)	(3.7)	(9.8)
Unrealized gains (losses)	6.9	(4.3)	10.0	(8.5)
Natural Gas | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	(1.6)	(1.3)	(3.3)	(2.6)
Unrealized gains (losses)	4.0	1.5	4.9	0.8
Electricity | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (losses) gains	0.0	(0.4)	0.0	(1.1)
Unrealized gains (losses)	0.0	1.3	0.0	1.7
Hedges Relating To Notes Member | Call Options | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	0.0	0.0	0.0	10.2
Hedges Relating To Notes Member | Bifurcated Conversion Feature | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized gains (losses)	$ 0.0	$ 0.0	$ 0.0	$ (10.2)