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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