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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
Derivative
Collateral posted	$ 2,400,000us-gaap_DerivativeLiabilityFairValueOfCollateral		$ 2,400,000us-gaap_DerivativeLiabilityFairValueOfCollateral
Counterparty A Paying 1.67% Interest
Derivative
Notional amount	25,000,000.0invest_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	200,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]	300,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.67PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]
Counterparty A Paying 1.65% Interest
Derivative
Notional amount	25,000,000.0invest_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	200,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]	300,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyAPaying1.65PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]
Counterparty B Paying 1.59% Interest
Derivative
Notional amount	25,000,000.0invest_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	100,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]	200,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying1.59PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]
Counterparty B Paying 2.14% Interest
Derivative
Notional amount	25,000,000.0invest_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	700,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]	600,000us-gaap_DerivativeLiabilities / us-gaap_DerivativeByNatureAxis = amg_CounterpartyBPaying2.14PercentInterestMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember	[1]
Other Liabilities
Derivative
Interest rate swaps at fair value	$ 1,200,000us-gaap_DerivativeLiabilities / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember		$ 1,400,000us-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.

X

- 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

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X

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

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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)

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- Definition

Fair value of securities pledged as collateral against derivative liabilities.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

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