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Income Taxes - Schedule of Income Tax Provisions by Controlling and Noncontrolling Interests (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Income Tax Provision
Deferred taxes	$ 34.1us-gaap_DeferredIncomeTaxExpenseBenefit		$ 15.1us-gaap_DeferredIncomeTaxExpenseBenefit
Provision for income taxes	69.6us-gaap_IncomeTaxExpenseBenefit		48.9us-gaap_IncomeTaxExpenseBenefit
Noncontrolling Interest
Income Tax Provision
Current tax	3.5us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		3.5us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
Deferred taxes	(0.2)us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		(0.2)us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
Provision for income taxes	3.3us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember		3.3us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember
Parent Company
Income Tax Provision
Current tax	32.0us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember		30.3us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Intangible-related deferred taxes	20.4amg_DeferredIncomeTaxExpenseRelatedToIntangibles / dei_LegalEntityAxis = us-gaap_ParentCompanyMember		16.9amg_DeferredIncomeTaxExpenseRelatedToIntangibles / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Other deferred taxes	13.9us-gaap_DeferredOtherTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember		(1.6)us-gaap_DeferredOtherTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Provision for income taxes	66.3us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember		45.6us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Income before income taxes (controlling interest)	$ 194.3amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / dei_LegalEntityAxis = us-gaap_ParentCompanyMember		$ 122.8amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
Effective tax rate attributable to controlling interests	34.10%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	[1]	37.10%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	[1]
[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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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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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.

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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

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

-Publisher FASB

-Name Accounting Standards Codification

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



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

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

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- Definition

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

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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.

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-Topic 740

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- Definition

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

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

-Publisher FASB

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