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Convertible Securities - Carrying Value and Principal Amount of Junior Convertible Securities (Details) - Junior convertible securities - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument |  |  |
Carrying Value | [1] | $ 309.9 | $ 307.5
Principal Amount at Maturity |  | $ 430.8 | $ 430.8
Debt instrument term |  | 20 years |
[1] | The carrying value is accreted to the principal amount at maturity over a remaining life of 20 years.

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- 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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Namespace Prefix: | amg_
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Balance Type: | credit
Period Type: | instant

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |