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Intangible Assets - Schedule of Changes in Goodwill (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Goodwill [Roll Forward]
Beginning balance (in dollars)	$ 2,652.8us-gaap_Goodwill
Goodwill acquired	0us-gaap_GoodwillAcquiredDuringPeriod
Foreign currency translation	(31.6)us-gaap_GoodwillTranslationAdjustments
Ending balance (in dollars)	2,621.2us-gaap_Goodwill
Institutional
Goodwill [Roll Forward]
Beginning balance (in dollars)	1,159.1us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Goodwill acquired	0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Foreign currency translation	(17.9)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Ending balance (in dollars)	1,141.2us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Mutual Fund
Goodwill [Roll Forward]
Beginning balance (in dollars)	1,125.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Goodwill acquired	0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Foreign currency translation	(3.5)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Ending balance (in dollars)	1,121.8us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
High Net Worth
Goodwill [Roll Forward]
Beginning balance (in dollars)	368.4us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Goodwill acquired	0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Foreign currency translation	(10.2)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Ending balance (in dollars)	$ 358.2us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_GoodwillRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of increase (decrease) from foreign currency translation adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=amg_HighNetWorthMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=amg_InstitutionalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=amg_MutualFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: