Note 11. Derivative Instruments (Detail) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative, Loss on Derivative	$ 9,053,000		$ (40,382,000)	$ 8,113,000	$ (38,907,000)
Derivative, Gain on Derivative	9,100,000			14,500,000
Unrealized Gain (Loss) on Derivatives	13,400,000			(22,600,000)
Payments for (Proceeds from) Derivative Instrument, Financing Activities		3,100,000
Concentrate Shipments Between Time of Sale and Settlement [Member]
Derivative Assets, Current	1,800,000			1,800,000
Derivative Liability, Fair Value, Net	1,900,000			1,900,000
Derivative Asset, Fair Value, Net	100,000			100,000
Derivative, Loss on Derivative	1,800,000			1,900,000
Forecasted Future Concentrate Shipments [Member] | Other Current Assets [Member]
Derivative Assets, Noncurrent	500,000			500,000
Forecasted Future Concentrate Shipments [Member] | Other NonCurrent Liabilities [Member]
Derivative Liabilities, Noncurrent	200,000			200,000
Forecasted Future Concentrate Shipments [Member]
Derivative Assets, Current	9,400,000			9,400,000
Derivative Liabilities, Noncurrent	100,000			100,000
Derivative Liabilities, Current	0.5			0.5
Gains on Settled Contracts [Member]
Derivative, Gain on Derivative	$ 4,300,000