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Derivatives Financial Instruments Derivative Financial Instruments - Schedule of Interest Rate Swap Agreements (Details) (Interest Rate Swap, USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Derivative |  |  |  |
Collateral posted | $ 2,400,000 us-gaap_DerivativeLiabilityFairValueOfCollateral |  | $ 2,400,000 us-gaap_DerivativeLiabilityFairValueOfCollateral |
Counterparty A Paying 1.67% Interest |  |  |  |
Derivative |  |  |  |
Notional amount | 25,000,000.0 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Paying | 1.67%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Interest rate swaps at fair value | 200,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1] | 300,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1]
Counterparty A Paying 1.65% Interest |  |  |  |
Derivative |  |  |  |
Notional amount | 25,000,000.0 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Paying | 1.65%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Interest rate swaps at fair value | 200,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1] | 300,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1]
Counterparty B Paying 1.59% Interest |  |  |  |
Derivative |  |  |  |
Notional amount | 25,000,000.0 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Paying | 1.59%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Interest rate swaps at fair value | 100,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1] | 200,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1]
Counterparty B Paying 2.14% Interest |  |  |  |
Derivative |  |  |  |
Notional amount | 25,000,000.0 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Paying | 2.14%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |  |
Interest rate swaps at fair value | 700,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1] | 600,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | [1]
Other Liabilities |  |  |  |
Derivative |  |  |  |
Interest rate swaps at fair value | $ 1,200,000 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  | $ 1,400,000 us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |
[1] | Aggregate fair values of $1.4 million and $1.2 million at December 31, 2014 and March 31, 2015, respectively, are presented within Other liabilities. The Company posted collateral with its counterparties of $2.4 million.

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition Fair value of securities pledged as collateral against derivative liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226013-175313 + Details
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- Details
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