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Affiliate Equity - Summary of Affiliate Equity Expense (Recognized and Unrecognized) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Affiliate Equity |  |  |
Non-controlling interest | $ 31.2 | $ 51.6 | $ 37.0
Total | 41.2 | 68.5 | 84.4
Non-Controlling Interests |  |  |
Affiliate Equity |  |  |
Total | 31.2 | 51.6 | 37.0
Unrecognized Affiliate Equity Expense |  |  |
Non-Controlling Interest | $ 70.7 | $ 51.9 | $ 41.6
Remaining Life | 5 years | 5 years | 6 years
Affiliate Partners |  |  |
Affiliate Equity |  |  |
Controlling interest | $ 10.0 | $ 16.9 | $ 47.4
Unrecognized Affiliate Equity Expense |  |  |
Controlling Interest | $ 31.3 | $ 22.4 | $ 29.5
Remaining Life | 4 years | 3 years | 3 years

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

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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: | credit
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- Definition [Line Items] for Affiliate Equity [Table] + References No definition available. + Details
Name: | amg_AffiliateEquityLineItems
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Affiliate Equity Unrecognized Compensation Expense, Attributable to Noncontrolling Interest + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
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- Definition Affiliate Equity, Unrecognized Compensation Expense, Weighted Average Period of Recognition + References No definition available. + Details
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- Definition Represents the compensation expenses related to Affiliate equity. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Unrecognized Affiliate Equity Expense [Abstract] + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Unrecognized Compensation Expenses Related to Affiliate Equity Attributable to Controlling Interest + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
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- Details
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Namespace Prefix: |
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- Details
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Period Type: |