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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
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Mar. 31, 2015 | Mar. 31, 2014
Income Tax Provision |  |  |  |
Deferred taxes | $ 34.1 us-gaap_DeferredIncomeTaxExpenseBenefit |  | $ 15.1 us-gaap_DeferredIncomeTaxExpenseBenefit |
Provision for income taxes | 69.6 us-gaap_IncomeTaxExpenseBenefit |  | 48.9 us-gaap_IncomeTaxExpenseBenefit |
Noncontrolling Interest |  |  |  |
Income Tax Provision |  |  |  |
Current tax | 3.5 us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |  | 3.5 us-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 | 3.3 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |  | 3.3 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_NoncontrollingInterestMember |
Parent Company |  |  |  |
Income Tax Provision |  |  |  |
Current tax | 32.0 us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  | 30.3 us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |
Intangible-related deferred taxes | 20.4 amg_DeferredIncomeTaxExpenseRelatedToIntangibles / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  | 16.9 amg_DeferredIncomeTaxExpenseRelatedToIntangibles / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |
Other deferred taxes | 13.9 us-gaap_DeferredOtherTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  | (1.6)us-gaap_DeferredOtherTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |
Provision for income taxes | 66.3 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  | 45.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |
Income before income taxes (controlling interest) | $ 194.3 amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent / dei_LegalEntityAxis = us-gaap_ParentCompanyMember |  | $ 122.8 amg_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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- 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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- 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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- 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) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 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 -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 Reference 4: 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)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 + Details
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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 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: 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 Reference 3: 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)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6510177 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- 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 Reference 2: 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)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6510177 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (g),(h) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- 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)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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