Note 11. Derivative Instruments (Detail) - Summary of base metals committed under forward sales contracts (USD $)	3 Months Ended	9 Months Ended
	Dec. 31, 2011	Sep. 30, 2012
Zinc [Member] | Settlements 2012 [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	9,600	8,525
Zinc [Member] | Settlements 2013 [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales		2,200
Zinc [Member] | Settlements 2012 [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	20,500
Zinc [Member] | Settlements 2013 [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	8,275	10,250
Lead [Member] | Settlements 2012 [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	2,600	3,400
Lead [Member] | Settlements 2012 [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	15,900	600
Lead [Member] | Settlements 2013 [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	11,150	16,850
Zinc [Member] | Settlements 2012 [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	$ 0.86	$ 0.88
Zinc [Member] | Settlements 2013 [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)		0.97
Zinc [Member] | Settlements 2012 [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.12
Zinc [Member] | Settlements 2013 [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.14	1.06
Lead [Member] | Settlements 2012 [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	0.89	0.96
Lead [Member] | Settlements 2012 [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.12	1.12
Lead [Member] | Settlements 2013 [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	$ 1.17	$ 1.12