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Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2012
Contingent payment arrangements |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 83.1
Contingent payment arrangements | Discounted cash flow | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Growth rate | 10.00%
Discount rate | 14.00%
Contingent payment arrangements | Discounted cash flow | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Growth rate | 18.00%
Discount rate | 18.00%
Affiliate equity repurchase obligations |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 14.9
Affiliate equity repurchase obligations | Discounted cash flow | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Growth rate | 8.00%
Discount rate | 16.00%
Affiliate equity repurchase obligations | Discounted cash flow | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Growth rate | 17.50%
Discount rate | 24.00%

X
- Definition Represents the interest rate used to determine 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 No definition available. + Details
Name: | amg_FairValueInputsDiscountRate
Namespace Prefix: | amg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | amg_FairValueInputsLiabilitiesQuantitativeInformationLineItems
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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 No definition available. + Details
Name: | us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant