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Share-Based Compensation - Restricted Stock (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Weighted average period for recognition (in years) | 3 years |  |
Restricted stock available for future grant under the Company's option plans (in shares) | 1.7 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |
Restricted Stock |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award fair value | $ 8.0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 34.1 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 18.1 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Vesting period |  | 6 years |
Market condition 1 percentage |  | 15.00%amg_SharebasedCompensationArrangementbySharebasedPaymentAwardMarketCondition1Percent / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Market condition 2 percentage |  | 25.00%amg_SharebasedPaymentAwardMarketCondition2RequisitePercentageIncreaseinStockPrice / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Market condition 3 percentage |  | 35.00%amg_SharebasedPaymentAwardMarketCondition3RequisitePercentageIncreaseinStockPrice / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Expected volatility |  | 31.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Risk-free interest rate |  | 2.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Dividend yield |  | 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Fair value of restricted stock awards |  | $ 19.4 amg_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsGrantsinPeriodAggregateIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Weighted average period for recognition (in years) |  | 4 years |
Restricted Stock | Minimum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Vesting period | 3 years |  |
Restricted Stock | Maximum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Vesting period | 8 years |  |

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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Aggregate Intrinsic Value + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Market Condition 1, Requisite Percentage Increase In Stock Price + References No definition available. + Details
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- Definition Share-based Payment Award, Market Condition 2, Requisite Percentage Increase in Stock Price + References No definition available. + Details
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- Definition Share-based Payment Award, Market Condition 3, Requisite Percentage Increase in Stock Price + References No definition available. + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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