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Commitments and Contingencies (Details) - USD ($) | Dec. 31, 2018 | Sep. 30, 2018
--- | --- | ---
Commitments and Contingencies |  |
Co-investment commitments in partnership |  | $ 130,400,000
Consolidated Affiliates |  |
Commitments and Contingencies |  |
Contingent liability |  | 12,700,000
Expected payments of contingent liability |  | 4,500,000
Present value of expected payments of contingent liabilities |  | 4,300,000
Consolidated Affiliates | Scenario, Forecast |  |
Commitments and Contingencies |  |
Expected payments of contingent liability | $ 2,500,000 |
Equity Method Investee |  |
Commitments and Contingencies |  |
Contingent liability |  | $ 150,000,000
Equity Method Investee | Scenario, Forecast |  |
Commitments and Contingencies |  |
Expected payments of contingent liability | $ 0 |

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
Name: | amg_CommitmentAndContingenciesLineItems
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount that the entity agreed to co-invest in certain investment partnerships where the entity serves as the general partner. + References No definition available. + Details
Name: | amg_InvestmentCommitmentAmount
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Other Commitment Expected Payments + References No definition available. + Details
Name: | amg_OtherCommitmentExpectedPayments
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Other Commitment Present Value of Expected Payments + References No definition available. + Details
Name: | amg_OtherCommitmentPresentValueofExpectedPayments
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
Name: | us-gaap_OtherCommitment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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