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Affiliate Equity Summary of affiliate equity compensation expense (Details) (Affiliated Entity [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Affiliated Entity [Member] |  |  |
Affiliate Equity [Line Items] |  |  |
Tax Benefit | $ 21.7 | $ 17.0 | $ 12.0
Affiliate Equity Compensation Expense | $ 72.3 | $ 60.4 | $ 38.1

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- Definition Affiliate Equity Compensation Expense, Including Portion Attributable to Noncontrolling Interest + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Affiliate Equity Compensation Expense, Tax Benefit (Expense) + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration