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Business Combinations - Schedule of Purchase Price Allocation for Investments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition
Goodwill	$ 2,652.8us-gaap_Goodwill	$ 2,341.7us-gaap_Goodwill	$ 2,355.2us-gaap_Goodwill
SouthernSun
Business Acquisition
Consideration paid	109.9us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Non-controlling interests	74.0us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Enterprise value	183.9amg_BusinessAcquisitionEnterpriseValue / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Acquired client relationships	125.3us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Deferred income taxes	0us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Tangible assets, net	6.3amg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredTangibleAssets / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Goodwill	52.3us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
Total	183.9us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember
River Road
Business Acquisition
Consideration paid	132.4us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Non-controlling interests	35.8us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Enterprise value	168.2amg_BusinessAcquisitionEnterpriseValue / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Acquired client relationships	64.1us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Deferred income taxes	0us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Tangible assets, net	4.5amg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredTangibleAssets / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Goodwill	99.6us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Total	168.2us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember
Veritas
Business Acquisition
Consideration paid	319.9us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Non-controlling interests	125.2us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Enterprise value	445.1amg_BusinessAcquisitionEnterpriseValue / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Acquired client relationships	275.6us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Deferred income taxes	(35.7)us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Tangible assets, net	12.6amg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredTangibleAssets / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Goodwill	192.6us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember
Total	$ 445.1us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember

X

- Definition

Represents the amount of Consideration Paid, Deferred Consideration, noncontrolling interest and Contingent payment obligations related to acquisition.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Business Combination, Recognized Identifiable Assets Acquired, Tangible Assets

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

This element represents the fair value of the noncontrolling interest in the acquiree at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)(1)

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X

- Definition

The amount of identifiable intangible assets recognized as of the acquisition date.

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- Definition

Amount of liabilities assumed at the acquisition date.

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- Definition

Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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X

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

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-Publisher FASB

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-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133



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