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Commitments and Contingencies (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2015 USD ($)
Commitments and Contingencies |
Co-investment commitments in partnership | $ 74.8
Expected payments to settle contingent obligations | 9.9
Net present value of expected payments to settle contingent obligations | 7.1
Prior Owner |
Commitments and Contingencies |
Reimbursable amount of investment commitments | 15.5
Consolidated Affiliates |
Commitments and Contingencies |
Acquisition agreements contingency liability | 72.1
Equity Method Affiliates |
Commitments and Contingencies |
Acquisition agreements contingency liability | $ 151.0

X
- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents the expected payment amounts during the period. + References No definition available. + Details
Name: | amg_BusinessCombinationContingentConsiderationArrangementsExpectedPayments
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the net present value of expected payments under contingent consideration arrangements recognized in connection with a business combination. + References No definition available. + Details
Name: | amg_BusinessCombinationContingentConsiderationArrangementsExpectedPaymentsNetPresentValue
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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 Represents the contractual obligation to reimburse the entity for co-investments in investment partnerships. + References No definition available. + Details
Name: | amg_ReimbursableAmountOfInvestmentCommitments
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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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: |
Period 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: |
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Period Type: |