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Business Combinations Narrative (Details) (Southern Sun Asset Management, LLC [Member])	0 Months Ended
	Mar. 31, 2014
Business Acquisition [Line Items]
Tax deduction period for goodwill and client relationships amortization expense	15 years
Institutional [Member]
Business Acquisition [Line Items]
Goodwill percentage attributable to segments	37.00%
Mutual Fund [Member]
Business Acquisition [Line Items]
Goodwill percentage attributable to segments	23.00%
High Net Worth [Member]
Business Acquisition [Line Items]
Goodwill percentage attributable to segments	40.00%

X

- Definition

Tax deduction period for goodwill and client relationships amortization expense.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of goodwill, attributable to business segment, arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed.

+ References

No definition available.

+ Details

Name:	amg_BusinessAcquisitionPurchasePriceAllocationGoodwillPercentage
Namespace Prefix:	amg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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