Derivative And Other Financial Instruments (Schedule of Derivative Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Asset
Fair value	$ 51.7	$ 35.3
Effect of counterparty netting	(18.1)	(27.2)
Net derivatives as classified in the balance sheet	33.6	8.1
Liability
Fair value	(18.5)	(36.2)
Effect of counterparty netting	18.1	27.2
Net derivatives as classified in the balance sheet	(0.4)	(9.0)
Metal
Asset
Fair value	48.1	33.5
Liability
Fair value	(18.5)	(36.0)
Energy Related Derivative
Asset
Fair value	0.2	0.2
Liability
Fair value	0.0	(0.1)
Currency
Asset
Fair value	3.4	1.6
Liability
Fair value	$ 0.0	$ (0.1)

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