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Note 10 - Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Note 10 - Derivative Instruments (Details) [Line Items]
Derivative, Gain on Derivative	$ 6.2	$ 1.5	$ 3.0
Unsettled Sales and Forecasted Future Concentrate Contracts [Member] \| Other Current Liabilities [Member]
Note 10 - Derivative Instruments (Details) [Line Items]
Derivative Liability, Current	0.3
Derivative Asset, Current	0.6
Unsettled Concentrate Sales Contracts [Member]
Note 10 - Derivative Instruments (Details) [Line Items]
Derivative, Gain on Derivative	6.2
Forecasted Future Concentrate Contracts [Member]
Note 10 - Derivative Instruments (Details) [Line Items]
Derivative, Gain on Derivative	8.3
Forecasted Future Concentrate Contracts [Member] \| Losses Realized On Settled Contracts [Member]
Note 10 - Derivative Instruments (Details) [Line Items]
Derivative, Gain on Derivative	$ 18.8

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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