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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% |

X
- Definition Represents the interest rate used to adjust for the time value of money assumed to determine the pension obligation and seniority premiums as of the year-end and the net cost. + References No definition available. + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to determine the pension obligation and seniority premiums as of year-end and the net cost. + References No definition available. + Details
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- Definition Represents the buyout payments outstanding to workers under defined benefit plans. + References No definition available. + Details
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- Definition Represents the plan obligation as a percentage of the entity's aggregate defined benefit obligation. + References No definition available. + Details
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- Definition Represents the percentage of interest rate of coupon one on which Bond Fund moved from overweight to neutral position. + References No definition available. + Details
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- Definition Represents the number of classes of grades in which Blackrock, Inc. has switched its exposure. + References No definition available. + Details
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- Definition Represents the percentage of total assets, the Money Fund intends to invest in bank obligations under normal market conditions. + References No definition available. + Details
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- Definition Represents the percentage of underweight versus the benchmark due to BlackRock, Inc.'s reduced exposure to agency mortgages at the end of the period. + References No definition available. + Details
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- Definition Represents the period of Bond Fund for which the interest rate or yield curve position was overweight. + References No definition available. + Details
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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + Details
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- Definition The aggregate percentage of the fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of total plan assets held as of the measurement date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + Details
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