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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, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Affiliate Equity |  |  |
Non-controlling interest | $ 51.6 | $ 37.0 | $ 15.9
Total | 62.0 | 66.2 | 50.6
Non-Controlling Interests |  |  |
Affiliate Equity |  |  |
Total | 51.6 | 37.0 | 15.9
Unrecognized Affiliate Equity Expense |  |  |
Non-Controlling Interest | $ 51.9 | $ 41.6 | $ 32.1
Remaining Life | 5 years | 6 years | 7 years
Affiliate Partners |  |  |
Affiliate Equity |  |  |
Controlling interest | $ 16.9 | $ 47.4 | $ 56.4
Tax benefit | (6.5) | (18.2) | (21.7)
Controlling interest, net | 10.4 | 29.2 | 34.7
Unrecognized Affiliate Equity Expense |  |  |
Controlling Interest | $ 22.4 | $ 29.5 | $ 36.1
Remaining Life | 3 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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- 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
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- Definition Affiliate Equity Compensation Expense, Tax Benefit (Expense) + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition [Line Items] for Affiliate Equity [Table] + References No definition available. + Details
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- Definition Affiliate Equity Unrecognized Compensation Expense, Attributable to Noncontrolling Interest + References No definition available. + Details
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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
Period Type: | duration

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- Definition Represents the compensation expenses related to affiliate equity, which were attributable to the controlling 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 Unrecognized Affiliate Equity Expense [Abstract] + References No definition available. + Details
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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
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- Details
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- Details
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Period Type: |