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Business and Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2014 Segment Channel
Segment Information [Abstract]
Number of operating segments	3us-gaap_NumberOfReportableSegments
Number of principal distribution channels	3amg_NumberOfPrincipalDistributionChannels
Office Equipment and Furniture and Fixtures | Minimum
Fixed Assets
Estimated useful life	3 years
Office Equipment and Furniture and Fixtures | Maximum
Fixed Assets
Estimated useful life	10 years
Computer software | Minimum
Fixed Assets
Estimated useful life	3 years
Computer software | Maximum
Fixed Assets
Estimated useful life	7 years
Building
Fixed Assets
Estimated useful life	39 years

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

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

X

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=amg_OfficeEquipmentAndFurnitureAndFixturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_SoftwareAndSoftwareDevelopmentCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: