Note 11. Derivative Instruments (Detail) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative, Loss on Derivative	$ (6,171,000)		$ (559,000)	$ (940,000)	$ 1,475,000
Derivative Assets, Noncurrent	5,700,000			5,700,000
Derivative, Gain on Derivative				900,000
Unrealized Gain (Loss) on Derivatives				9,300,000
Payments for (Proceeds from) Derivative Instrument, Financing Activities		3,100,000
Concentrate Shipments Between Time of Sale and Settlement [Member]
Derivative Assets, Current	200,000			200,000
Derivative Liability, Fair Value, Net	300,000			300,000
Derivative, Loss on Derivative				100,000
Forecasted Future Concentrate Shipments [Member]
Derivative Assets, Current	17,500,000			17,500,000
Derivative Liability, Fair Value, Net	19,000			19,000
Gains on Settled Contracts [Member]
Derivative, Gain on Derivative				$ 10,200,000