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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%

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

- 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

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_FairValueInputsLiabilitiesQuantitativeInformationLineItems
Namespace Prefix:	us-gaap_
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

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

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant