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Commitments and Contingencies (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Jun. 30, 2021
Commitments and Contingencies |  |
Co-investment commitments in partnership |  | $ 128.5
Equity Method Investee |  |
Commitments and Contingencies |  |
Total contingent payments |  | 104.4
Contingent payments in year one |  | $ 77.5
Noncontrolling interest, sell of ownership percentage | 5.00% |
Undisclosed Affiliate | Equity Method Investee |  |
Commitments and Contingencies |  |
Ownership percentage by minority owner |  | 14.00%
Payments for Achievement of Financial Targets |  |
Commitments and Contingencies |  |
Total contingent payments |  | $ 188.0
Contingent payments in year one |  | 40.5
Contingent payments after year one |  | 147.5
Expected payments |  | 13.0
Payment contingent on not meeting financial targets (up to) |  | 12.5
Payment contingent on exercise of option to reduce ownership percentage |  | $ 25.0

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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
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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, Contingent Consideration, Asset + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Other Commitment, Contingent Consideration, Option To Reduce Ownership Interest, Asset + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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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, To Be Paid, After Year One + References No definition available. + Details
Name: | amg_OtherCommitmentToBePaidAfterYearOne
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of commitment classified as other to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
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Balance Type: | credit
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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

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