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Affiliate Equity - Summary of Affiliate Recognized and Unrecognized Expense (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Affiliate Equity [Line Items] |  |  |
Affiliate equity expense | $ 16.6 | $ 10.7 |
Non-controlling Interests |  |  |
Affiliate Equity [Line Items] |  |  |
Non-controlling interests | $ 13.8 | 8.5 |
Unrecognized Affiliate Equity Expense [Abstract] |  |  |
Remaining Life | 6 years |  | 6 years
Non-controlling Interests | $ 126.0 |  | $ 124.6
Affiliated Entity |  |  |
Affiliate Equity [Line Items] |  |  |
Controlling interest | 2.8 | $ 2.2 |
Unrecognized Affiliate Equity Expense [Abstract] |  |  |
Controlling Interest | $ 42.5 |  | $ 40.9
Remaining Life | 4 years |  | 4 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
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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
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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, Remaining Life + References No definition available. + Details
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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
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- Definition Unrecognized Affiliate Equity Expense [Abstract] + References No definition available. + Details
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Namespace Prefix: | amg_
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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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: |
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- Details
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Period Type: |