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Senior Bank Debt (Details) (USD $) In Millions, unless otherwise specified				9 Months Ended
	Sep. 30, 2012 Amended credit facility	Sep. 30, 2012 Revolving credit facility	Dec. 31, 2011 Revolving credit facility	Sep. 30, 2012 Term loan
Senior Bank Debt
Maximum borrowing capacity	$ 1,075.0	$ 825.0		$ 250.0
Portion of the principal balance maturing in January 2015		30.0
Principal balance maturing in November 2016		795.0
Option to increase the borrowing capacity under revolving credit facility		75.0		250.0
Line of credit facility amount outstanding		$ 445.0	$ 250.0
Term of debt				5 years

X

- Definition

Represents the expected term of the debt instrument.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Represents the option to increase borrowings on 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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- Details

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X

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 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



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 10

-Subparagraph b

-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.

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- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 10

-Subparagraph b

-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.

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