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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