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Benefit Plans (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 plan | Dec. 31, 2012 | Dec. 31, 2011
Compensation and Retirement Disclosure [Abstract] |  |  |
Number of defined contribution plans | 3 |  |
Consolidated expenses related to benefit plans | $ 14.1 | $ 12.7 | $ 12.0

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- Definition The number of defined contribution plans sponsored by the entity. + References No definition available. + Details
Name: | amg_DefinedContributionPlansNumber
Namespace Prefix: | amg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration