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Intangible Assets - Schedule of Changes in the Components of Acquired Client Relationships (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Total |  |
Beginning balance, net (in dollars) |  | $ 1,778.4 us-gaap_IntangibleAssetsNetExcludingGoodwill
Ending balance, net (in dollars) | 1,724.7 us-gaap_IntangibleAssetsNetExcludingGoodwill | 1,778.4 us-gaap_IntangibleAssetsNetExcludingGoodwill
Acquired Client Relationships |  |
Definite-lived |  |
Beginning balance, gross (in dollars) | 1,255.1 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Beginning balance, accumulated amortization | (565.0)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Beginning balance, net (in dollars) | 690.1 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
New investments | 0 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Amortization and impairments | (27.8)us-gaap_ImpairmentOfIntangibleAssetsFinitelived / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Foreign currency translation | (5.1)us-gaap_FiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Ending balance, gross (in dollars) | 1,250.0 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Ending balance, accumulated amortization | (592.8)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Ending balance, net (in dollars) | 657.2 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Indefinite-lived |  |
Beginning balance, net (in dollars) | 1,088.3 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
New investments | 0 us-gaap_IndefinitelivedIntangibleAssetsAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Amortization and impairments | 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Foreign currency translation | (20.8)us-gaap_IndefiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Ending balance, net (in dollars) | 1,067.5 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Total |  |
Beginning balance, net (in dollars) | 1,778.4 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
New investments | 0 amg_IntangibleAssetsNetExcludingGoodwillAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Amortization and impairments | (27.8)us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Foreign currency translation | (25.9)amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Ending balance, net (in dollars) | $ 1,724.7 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |

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