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Forward Equity Sale Agreement (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012
Forward Equity Sale Agreement
Notional amount sold	$ 77.0amg_ForwardEquityContractsSharesSoldAmount
Average share price (in dollars per share)	$ 185.56amg_ForwardEquityContractsSharesSoldAverageSharePrice
Derivative remaining notional amount		252.8amg_DerivativeRemainingNotionalAmount
Equity Contract
Forward Equity Sale Agreement
Notional amount		70.2invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_EquityContractMember
Amended and Restated Forward Equity Sale Agreement
Forward Equity Sale Agreement
Notional amount sold			147.2amg_ForwardEquityContractsSharesSoldAmount / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember
Average share price (in dollars per share)		$ 198.71amg_ForwardEquityContractsSharesSoldAverageSharePrice / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember	$ 121.37amg_ForwardEquityContractsSharesSoldAverageSharePrice / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember
Notional amount		$ 400.0invest_DerivativeNotionalAmount / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember

X

- Definition

Derivative, Remaining Notional Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	amg_ForwardContractLineItems
Namespace Prefix:	amg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the sum of common stock sold under the forward equity sale agreements.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsSharesSoldAmount
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Represents the average share price of common stock sold under the forward equity sale agreements.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsSharesSoldAverageSharePrice
Namespace Prefix:	amg_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

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

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

X

- Details

Name:	amg_ForwardContractAxis=amg_AmendedAndRestatedEquityForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DerivativeByNatureAxis=us-gaap_EquityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: