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Affiliate Equity - Schedule of the Changes in Redeemable Non-controlling Interests (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Changes in redeemable non-controlling interests during the period |  |
Beginning balance | $ 641.9 us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount | $ 477.5 us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount
Transactions in Redeemable non-controlling interests | (61.7)us-gaap_PaymentsForRepurchaseOfRedeemableNoncontrollingInterest | (52.5)us-gaap_PaymentsForRepurchaseOfRedeemableNoncontrollingInterest
Changes in redemption value | 65.3 amg_RedeemableNoncontrollingInterestChangeInRedemptionValue | 216.9 amg_RedeemableNoncontrollingInterestChangeInRedemptionValue
Ending balance | $ 645.5 us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount | $ 641.9 us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount

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- Definition Increase (decrease) in redeemable non-controlling interests during the period as a result of a change in the redemption value. + References No definition available. + Details
Name: | amg_RedeemableNoncontrollingInterestChangeInRedemptionValue
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Definition The cash outflow during the period for redemption of redeemable noncontrolling interests. + References No definition available. + Details
Name: | us-gaap_PaymentsForRepurchaseOfRedeemableNoncontrollingInterest
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition As of the reporting date, the aggregate carrying amount of all noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. This item includes noncontrolling interest holder's ownership (or holders' ownership) regardless of the type of equity interest (common, preferred, other) including all potential organizational (legal) forms of the investee entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph 14 -URI http://asc.fasb.org/extlink&oid=27011957&loc=SL6540498-122764 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph (16)(c) -URI http://asc.fasb.org/extlink&oid=27011957&loc=SL6540498-122764 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph 15 -URI http://asc.fasb.org/extlink&oid=27011957&loc=SL6540498-122764 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph (12)(c) -URI http://asc.fasb.org/extlink&oid=27011957&loc=SL6540498-122764 + Details
Name: | us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant