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Segment Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2013 Channel Segment	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of operating segments	3
Number of principal distribution channels	3
Revenue attributable to clients domiciled outside the United States (as a percent)	38.00%	41.00%	39.00%

X

- Definition

Number of principal distribution channels.

+ References

No definition available.

+ Details

Name:	amg_NumberOfPrincipalDistributionChannels
Namespace Prefix:	amg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of total revenues from external customers attributed to foreign countries from which the entity derives revenues.

+ References

No definition available.

+ Details

Name:	amg_PercentageOfRevenueFromExternalCustomersAttributedToForeignCountries
Namespace Prefix:	amg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration