Name:	amg_BusinessCombinationContingentConsiderationArrangementsExpectedPayments
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

For contingent consideration arrangements recognized in connection with a business combination, this element represents the expected payment amounts within the current fiscal period.

No definition available.

Name:	amg_BusinessCombinationContingentConsiderationArrangementsExpectedPaymentsInFiscalYear
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the net present value of expected payments under contingent consideration arrangements recognized in connection with a business combination.

No definition available.

Name:	amg_BusinessCombinationContingentConsiderationArrangementsExpectedPaymentsNetPresentValue
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

For contingent consideration arrangements recognized in connection with a business combination, this element represents the recognized gain during the period.

No definition available.

Name:	amg_BusinessCombinationContingentConsiderationArrangementsRecognizedGain
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

For contingent consideration arrangements recognized in connection with a business combination, this element represents the recognized gain attributable to the controlling interest during the period.

No definition available.

Name:	amg_BusinessCombinationContingentConsiderationArrangementsRecognizedGainAttributableToControllingInterest
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	amg_CommitmentAndContingenciesLineItems
Namespace Prefix:	amg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount that the entity agreed to co-invest in certain investment partnerships where the entity serves as the general partner.

No definition available.

Name:	amg_InvestmentCommitmentAmount
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the contractual obligation to reimburse the entity for co-investments in investment partnerships.

No definition available.

Name:	amg_ReimbursableAmountOfInvestmentCommitments
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph g

-Clause 3

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6911361&loc=d3e6927-128479