Note 10 - Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	2 Months Ended	12 Months Ended
	Mar. 01, 2012	Dec. 31, 2012	Dec. 31, 2011
Derivative Assets, Current		$ 5.6
Derivative, Gain on Derivative		10.5	7.1
Derivative Liabilities, Noncurrent		2.5
Payments for (Proceeds from) Derivative Instrument, Financing Activities	3.1
Forecasted Future Concentrate Contracts [Member]
Derivative Assets, Current		2.3
Unsettled Concentrate Sales Contracts [Member]
Derivative, Gain on Derivative		1.3
Net From Current Asset Balance [Member]
Derivative Liability, Fair Value, Net		1.0
Net From Non Current Liability Balance [Member]
Derivative Liability, Fair Value, Net		0.1
Losses Realized On Settled Contracts [Member]
Derivative, Gain on Derivative		$ 18.4