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BENEFIT PLANS: (Details 2) (USD $)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
class
Y
Dec. 31, 2010
Dec. 31, 2009
Peru Defined Benefit Pension Plans
      
Benefit plans      
Net asset value (in dollars per share) $ 1.00    
Target duration of funds invested in publicly traded, investment grade securities (in years) 1.5    
Percentage of total assets, the Money Fund intends to invest in bank obligations under normal market conditions 25.00%    
Maturity period of 4% coupon on which Bond Fund has an underweight position (in years) 30    
Interest rate of coupon on which Bond Fund has an underweight position (as a percentage) 4.00%    
Maturity period of 4.5% coupon on which Bond Fund has an overweight position (in years) 30    
Interest rate of coupon on which Bond Fund is overweight (as a percent) 4.50%    
Interest rate of coupon one on which Bond Fund moved from very overweight to neutral position (as a percent) 3.00%    
Interest rate of coupon two on which Bond Fund moved from very overweight to neutral position (as a percent) 3.50%    
Number of classes of grades in which Blackrock has switched its exposure 2    
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Discount rate (as a percent) 3.95% 3.95% 5.55%
Expected long-term rate of return on plan asset (as a percent) 4.50% 4.50% 4.50%
Peru Defined Benefit Pension Plans | Minimum
      
Benefit plans      
Period of Bond Fund for which interest rate or yield curve position was overweight 7    
Percentage of underweight versus the benchmark due to BlackRock's reduced exposure to agency mortgages 2.00%    
Peru Defined Benefit Pension Plans | Maximum
      
Benefit plans      
Period of Bond Fund for which interest rate or yield curve position was overweight 10    
Percentage of underweight versus the benchmark due to BlackRock's reduced exposure to agency mortgages 3.00%    
Peru Post-retirement Health Care Plan
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Discount rate (as a percent) 3.95% 5.00%  
Minera Mexico Defined Benefit Pension Plans
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Discount rate (as a percent) 7.50% 7.50% 8.00%
Expected long-term rate of return on plan asset (as a percent) 7.50% 7.50% 8.00%
Number of financial institutions managing plan assets 3    
Plan obligations as a percentage of benefit obligation 30.00%    
Asset allocation (as a percent) 100.00% 100.00%  
Expected employer contribution in next fiscal year $ 7.3    
Pending payments to former Buenavista workers $ 3.4    
Minera Mexico Defined Benefit Pension Plans | Equity securities
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Asset allocation (as a percent) 74.00% 71.00%  
Minera Mexico Defined Benefit Pension Plans | Equity securities | Parent Company (Grupo Mexico) common shares.
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Asset allocation (as a percent) 74.00% 71.00%  
Minera Mexico Defined Benefit Pension Plans | Treasury bills
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Asset allocation (as a percent) 26.00% 29.00%  
Minera Mexico Post-retirement health care plan
      
Assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost in the ensuing year      
Discount rate (as a percent) 7.50% 7.50%