Note 11. Derivative Instruments (Detail) - Summary of base metals committed under forward sales contracts (USD $)	3 Months Ended	6 Months Ended
	Dec. 31, 2011	Jun. 30, 2012
Settlements 2012 [Member] | Zinc [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	20,500	5,925
Settlements 2012 [Member] | Lead [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	15,900	2,500
Settlements 2012 [Member] | Zinc [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	$ 1.12	$ 1.11
Settlements 2012 [Member] | Lead [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.12	1.12
Settlements 2013 [Member] | Zinc [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	8,275	10,375
Settlements 2013 [Member] | Lead [Member]
Contracts on forecasted sales
Metric tonnes under contract - forecasted sales	11,150	13,850
Settlements 2013 [Member] | Zinc [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.14	1.10
Settlements 2013 [Member] | Lead [Member]
Contracts on forecasted sales
Average price per pound - forecasted sales (in Dollars)	1.17	1.14
Settlements 2012 [Member] | Zinc [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	9,600	9,000
Settlements 2012 [Member] | Lead [Member]
Contracts on provisional sales
Metric tonnes under contract - provisional sales	2,600	2,300
Settlements 2012 [Member] | Zinc [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	0.86	0.86
Settlements 2012 [Member] | Lead [Member]
Contracts on provisional sales
Average price per pound - provisional sales (in Dollars)	$ 0.89	$ 0.84