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Fair Value Measurements Fair Value Measurements - Summary of other financial liabilities not carried at fair value (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$ 340.0	$ 340.0
Senior convertible securities	0	450.1
Junior convertible trust preferred securities	517.9	515.5
Level 2 [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	340.0	340.0
Senior convertible securities	0	450.1
Junior convertible trust preferred securities	517.9	515.5
Level 2 [Member] | Fair Market Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	324.2	351.8
Senior convertible securities	0	510.6
Junior convertible trust preferred securities	$ 870.2	$ 719.9

X

- Definition

Carrying amount of junior long-term convertible debt as of the balance sheet date, net of the amount due in the next twelve months. The debt is convertible, at any time, into a defined number of shares of common stock. Holders of the junior convertible trust preferred securities have no rights to put these securities.

+ References

No definition available.

+ Details

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X

- Definition

Carrying amount of senior long-term convertible debt as of the balance sheet date, net of the amount due in the next twelve months. The debt is convertible into a defined number of shares of common stock upon the occurrence of certain events. The holders may put these securities to the company.

+ References

No definition available.

+ Details

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

X

- Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: 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)

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



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

+ Details

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