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Affiliate Equity - Recognized and Unrecognized Compensation Expense (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Affiliate Equity Expense |  |  |
Total | $ 40.5 | $ 56.4 | $ 50.0
Non-controlling Interests |  |  |
Affiliate Equity Expense |  |  |
Non-controlling interests | $ 30.9 | $ 39.7 | $ 36.8
Unrecognized Affiliate Equity Expense |  |  |
Remaining Life | 6 years | 6 years | 6 years
Non-controlling Interests | $ 124.6 | $ 118.3 | $ 95.9
Controlling Interest |  |  |
Affiliate Equity Expense |  |  |
Controlling interest | 9.6 | 16.7 | 13.2
Unrecognized Affiliate Equity Expense |  |  |
Controlling Interest | $ 40.9 | $ 38.7 | $ 33.3
Remaining Life | 4 years | 5 years | 5 years

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

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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
Period Type: | duration

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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
Balance Type: | debit
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- Definition Affiliate Equity, Unrecognized Compensation Expense, Weighted Average Period of Recognition + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
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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
Period Type: | duration

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- Definition Unrecognized Affiliate Equity Expense [Abstract] + References No definition available. + Details
Name: | amg_UnrecognizedAffiliateEquityExpenseAbstract
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_NoncontrollingInterestMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |