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Forward Equity - Narrative (Details) (Forward equity sales [Member], USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013 | Apr. 30, 2014 Subsequent Event [Member]
--- | --- | --- | ---
Derivative [Line Items] |  |  |
Notional amount |  | $ 70.2 | $ 252.8
Average forward price | 199.19 | 123.09 | 198.58
Notional amount settled | $ 15.1 |  |

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- Definition Derivative, Notional Amount, Settled + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts. + References No definition available. + Details
Name: | us-gaap_DerivativeAverageForwardPrice
Namespace Prefix: | us-gaap_
Data Type: | us-types:perUnitItemType
Balance Type: | na
Period Type: | instant

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