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Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2014 | Mar. 31, 2014 Pantheon's prior owner [Member] | Dec. 31, 2014 Scenario, Forecast [Member]
Commitments and Contingencies |  |  |
Co-investment commitments in partnership | $ 73.1 |  |
Reimbursable amount of investment commitments |  | 27.4 |
Acquisition agreements contingency liability | 474.1 |  |
Expected payments related to equity method investments | 151.0 |  |
Expected payments to settle contingent obligations | 75.0 |  | 0
Net present value of expected payments to settle contingent obligations | $ 53.2 |  |

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

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

X
- 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

X
- Details
Name: | amg_CommitmentAndContingenciesLineItems
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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

X
- 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

X
- 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=7488404&loc=d3e6927-128479 + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant