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Derivative Financial Instruments - Schedule of Interest Rate Swap Agreements (Details) (Interest Rate Swap, USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Counterparty A Paying 1.67% Interest
Derivative
Notional amount	$ 25.0invest_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
Counterparty A Paying 1.65% Interest
Derivative
Notional amount	25.0invest_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
Counterparty B Paying 1.59% Interest
Derivative
Notional amount	25.0invest_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
Counterparty B Paying 2.14% Interest
Derivative
Notional amount	$ 25.0invest_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

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

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration