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FINANCIAL INSTRUMENTS (Details)
3 Months Ended 6 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2011
USD ($)
Jun. 30, 2010
USD ($)
Jun. 30, 2011
USD ($)
Jun. 30, 2010
USD ($)
Jun. 30, 2011
Call Options Written
USD ($)
Jun. 30, 2010
Call Options Written
USD ($)
Jun. 30, 2011
Call Options Written
Zinc
MT
Jun. 30, 2010
Call Options Written
Zinc
MT
Jun. 30, 2011
Put options purchased
Zinc
MT
Jun. 30, 2010
Put options purchased
Zinc
MT
Jun. 30, 2011
Foreign exchange forward contract
USD ($)
Jun. 30, 2011
Foreign exchange forward contract
USD ($)
Mar. 31, 2011
Foreign exchange forward contract
USD ($)
Mar. 31, 2011
Foreign exchange forward contract
CAD
Jun. 30, 2011
Zinc
USD ($)
Jun. 30, 2010
Zinc
USD ($)
Jun. 30, 2011
Zinc
USD ($)
Jun. 30, 2010
Zinc
USD ($)
Jun. 30, 2011
Silver
USD ($)
Jun. 30, 2011
Silver
USD ($)
Derivative                                        
Net premium payable to counterparty                                        
Zinc options (in metric tonnes)             20,000 15,000 20,000 15,000                    
Strike price for option (in dollars per metric tonne)             2,500 2,500 2,200 2,200                    
Options expiring each month, beginning from February 28 (2011) or March 31 (2010) (in metric tonnes)             2,000 1,500 2,000 1,500                    
Limit for participation, zinc prices set by zero-cost collar strategy (per metric tonne)                             2,500 2,500 2,500 2,500    
Gain (Loss) on derivative financial instruments 981,000 6,395,000 2,332,000 7,162,000             900,000       100,000 1,300,000        
Unrealized mark-to-market gain                                 700,000 3,900,000    
Exchange rate under foreign exchange forward contract (per US dollar)                           0.99            
Amount of expenditures hedged                         90,000,000              
Amount of expenditures hedged, expiring each month, beginning from April 2011                         10,000,000              
Call option premiums generated         2,500,000 2,000,000                            
Gain (loss) recognized in consolidated statement of income and comprehensive income                       $ 1,400,000             $ 1,300,000 $ 3,400,000