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Business and Summary of Significant Accounting Policies (Details) | 12 Months Ended
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Dec. 31, 2014 Segment Channel
Segment Information [Abstract] |
Number of operating segments | 3 us-gaap_NumberOfReportableSegments
Number of principal distribution channels | 3 amg_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: |