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Subsequent Events (Details) (USD $)	1 Months Ended				1 Months Ended
	Jul. 31, 2011 agreement	Jun. 30, 2012 Revolving credit facility	Jul. 31, 2012 Blue Mountain Capital Management	Jul. 31, 2012 Additional commitment Revolving credit facility	Aug. 31, 2012 Issuance of debt 2042 Senior Notes institution	Aug. 31, 2012 Forward equity sale agreement agreement
Subsequent Events
Increase in borrowing capacity				$ 75,000,000
Maximum borrowing capacity		750,000,000		825,000,000
Value of managed assets			7,000,000,000
Issuance of debt					200,000,000
Interest rate (as a percent)					6.375%
Number of financial institutions					3
Number of agreements under the forward equity sale agreements	2					2
Value of common stock to sell under each agreement, maximum	$ 300,000,000					$ 400,000,000

X

- Definition

Represents the maximum value of common stock under each forward equity sale agreements to sell shares of the entity's common stock.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the number of the agreements under the forward equity sale agreements, entered by the entity.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:integerItemType
Balance Type:	na
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X

- Definition

Represents the increase in borrowing capacity under the credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

Represents number of financial institutions.

+ References

No definition available.

+ Details

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X

- Definition

The value of assets that are managed by a company

+ References

No definition available.

+ Details

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

X

- Definition

Interest rate stated in the contractual debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	na
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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	credit
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- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

No definition available.

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