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Fair Value Measurements (Details 2) (Discounted cash flow, USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2013 | Dec. 31, 2012
Contingent payment arrangements |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Fair value Liabilities | 40.5 | 31.0
Contingent payment arrangements | Minimum |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Growth rate (as a percent) | 5.50% | 6.00%
Discount rate (as a percent) | 14.00% | 14.00%
Contingent payment arrangements | Maximum |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Growth rate (as a percent) | 11.00% | 12.00%
Discount rate (as a percent) | 18.00% | 18.00%
Affiliate equity repurchase obligations |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Fair value Liabilities | 21.4 | 9.4
Affiliate equity repurchase obligations | Minimum |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Growth rate (as a percent) | (10.00%) | (10.00%)
Discount rate (as a percent) | 15.00% | 15.00%
Affiliate equity repurchase obligations | Maximum |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Growth rate (as a percent) | 13.50% | 17.00%
Discount rate (as a percent) | 24.00% | 24.00%

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- Definition Represents percentage of assumed growth rates, used as an input to measure fair value. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant