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Commitments and Contingencies (Details) - USD ($) | Dec. 31, 2019 | Jun. 30, 2019
--- | --- | ---
Commitments and Contingencies |  |
Co-investment commitments in partnership | $ 127,200,000 |
Equity Method Investee |  |
Commitments and Contingencies |  |
Contingent payments through 2021 | 150,000,000.0 |
Contingent payments through 2022 | 40,000,000.0 |
Other commitment, expected payments | 25,000,000 |
Expected payments in 2020 | $ 0 |
Equity Method Investee | Non-US |  |
Commitments and Contingencies |  |
Noncontrolling interest, sale of ownership percentage | 5.00% | 5.00%

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
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Data Type: | xbrli:stringItemType
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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
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Noncontrolling Interest, Notification of Possible Sale of Noncontrolling Interest, Percentage + References No definition available. + Details
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- Definition Other Commitment Expected Payments + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Minimum amount of other commitment maturing in the next fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Minimum amount of other commitment maturing in the second fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Minimum amount of other commitment maturing in the third fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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- Details
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- Details
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