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Forward Equity Sale Agreement (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2012
Forward Equity Sale Agreement |  |  |
Notional amount sold | $ 77.0 amg_ForwardEquityContractsSharesSoldAmount |  |
Average share price (in dollars per share) | $ 185.56 amg_ForwardEquityContractsSharesSoldAverageSharePrice |  |
Derivative remaining notional amount |  | 252.8 amg_DerivativeRemainingNotionalAmount |
Equity Contract |  |  |
Forward Equity Sale Agreement |  |  |
Notional amount |  | 70.2 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_EquityContractMember |
Amended and Restated Forward Equity Sale Agreement |  |  |
Forward Equity Sale Agreement |  |  |
Notional amount sold |  |  | 147.2 amg_ForwardEquityContractsSharesSoldAmount / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember
Average share price (in dollars per share) |  | $ 198.71 amg_ForwardEquityContractsSharesSoldAverageSharePrice / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember | $ 121.37 amg_ForwardEquityContractsSharesSoldAverageSharePrice / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember
Notional amount |  | $ 400.0 invest_DerivativeNotionalAmount / amg_ForwardContractAxis = amg_AmendedAndRestatedEquityForwardMember |

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

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- Details
Name: | amg_ForwardContractLineItems
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the sum of common stock sold under the forward equity sale agreements. + References No definition available. + Details
Name: | amg_ForwardEquityContractsSharesSoldAmount
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the average share price of common stock sold under the forward equity sale agreements. + References No definition available. + Details
Name: | amg_ForwardEquityContractsSharesSoldAverageSharePrice
Namespace Prefix: | amg_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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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
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | amg_ForwardContractAxis=amg_AmendedAndRestatedEquityForwardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeByNatureAxis=us-gaap_EquityContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |