Note 11. Derivative Instruments (Detail) - Summary of base metals committed under forward sales contracts (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Settlements 2012 [Member] | Zinc [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	6,250	9,600
Settlements 2012 [Member] | Lead [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	1,300	2,600
Settlements 2012 [Member] | Zinc [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	$ 0.94	$ 0.86
Settlements 2012 [Member] | Lead [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	0.96	0.89
Settlements 2012 [Member] | Zinc [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	11,850	20,500
Settlements 2012 [Member] | Lead [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	5,100	15,900
Settlements 2012 [Member] | Zinc [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.12	1.12
Settlements 2012 [Member] | Lead [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.13	1.12
Settlements 2013 [Member] | Zinc [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	8,675	8,275
Settlements 2013 [Member] | Lead [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	13,250	11,150
Settlements 2013 [Member] | Zinc [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.13	1.14
Settlements 2013 [Member] | Lead [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	$ 1.15	$ 1.17