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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