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Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Dec. 31, 2012 | Sep. 30, 2012
Commitments and Contingencies |  |
Acquisition agreements contingency liability |  | $ 755.5
Expected payments to settle contingent obligations | 85.4 | 279.5
Net present value of expected payments to settle contingent obligations |  | 69.2
Expected payments related to equity method investments |  | 182.6
Recognized gain under past acquisition agreements |  | 57.3
Recognized gain attributable to the controlling interest |  | 39.6
Pantheon's prior owner |  |
Commitments and Contingencies |  |
Reimbursable amount of investment commitments |  | 39.5
Pantheon |  |
Commitments and Contingencies |  |
Co-investment commitments in partnership |  | $ 75.1

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
- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents the recognized gain during the period. + References No definition available. + Details
Name: | amg_BusinessCombinationContingentConsiderationArrangementsRecognizedGain
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 recognized gain attributable to the controlling interest during the period. + References No definition available. + Details
Name: | amg_BusinessCombinationContingentConsiderationArrangementsRecognizedGainAttributableToControllingInterest
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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141R -Paragraph 68 -Subparagraph g -Clause 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant