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Income Taxes - Schedule of Income Tax Provision Attributable to Controlling and Non-Controlling Interests (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income tax provision
Current tax	$ 165.1us-gaap_CurrentIncomeTaxExpenseBenefit		$ 166.4us-gaap_CurrentIncomeTaxExpenseBenefit		$ 72.7us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred taxes	62.8us-gaap_DeferredIncomeTaxExpenseBenefit		27.7us-gaap_DeferredIncomeTaxExpenseBenefit		11.1us-gaap_DeferredIncomeTaxExpenseBenefit
Provision for income taxes	227.9us-gaap_IncomeTaxExpenseBenefit		194.1us-gaap_IncomeTaxExpenseBenefit		83.8us-gaap_IncomeTaxExpenseBenefit
Controlling Interests
Income tax provision
Current tax	149.8us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		153.1us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		61.0us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember
Intangible-related deferred taxes	47.8amg_DeferredIncomeTaxExpenseRelatedToIntangibles / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		38.1amg_DeferredIncomeTaxExpenseRelatedToIntangibles / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		22.7amg_DeferredIncomeTaxExpenseRelatedToIntangibles / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember
Other deferred taxes	15.8us-gaap_DeferredOtherTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		(6.2)us-gaap_DeferredOtherTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		(12.1)us-gaap_DeferredOtherTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember
Provision for income taxes	213.4us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		185.0us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		71.6us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember
Income before income taxes (controlling interest)	665.5amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		545.5amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember		245.6amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember
Effective tax rate attributable to controlling interests	32.10%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember	[1]	33.90%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember	[1]	29.20%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / us-gaap_StatementEquityComponentsAxis = us-gaap_ParentMember	[1]
Non-Controlling Interests
Income tax provision
Current tax	15.3us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		13.3us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		11.7us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
Deferred taxes	(0.8)us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		(4.2)us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		0.5us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
Provision for income taxes	$ 14.5us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		$ 9.1us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		$ 12.2us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
[1] Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).

X

- Definition

The component of income tax expense for the period representing the tax effect of the difference between amortization of intangibles for book purposes and amortization of intangibles for tax purposes.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

This element represents the income or loss from continuing operations, before income taxes, which is attributable to the controlling interest.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

A ratio calculated by dividing the amount of income tax expense attributable to the controlling interest by pretax income from continuing operations attributable to the controlling interest.

+ References

No definition available.

+ Details

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X

- Definition

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 6.I.7)

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Current Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6509736



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Publisher SEC

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-Number 210

-Section 08

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section I

-Subsection 7

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- Definition

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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-Publisher FASB

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-Glossary Deferred Tax Expense (or Benefit)

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of other deferred income tax expense (benefit) pertaining to income (loss) from continuing operations. For example, but not limited to, acquisition-date income tax benefits or expenses recognized from changes in the acquirer's valuation allowance for its previously existing deferred tax assets resulting from a business combination and adjustments to beginning-of-year balance of a valuation allowance because of a change in circumstance causing a change in judgment about the realizability of the related deferred tax asset in future periods.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 6.I.7)

-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817



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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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-Glossary Deferred Tax Expense (or Benefit)

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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Balance Type:	debit
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X

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

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-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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