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Goodwill and Acquired Client Relationships - Schedule of Changes in, and the Components of, Acquired Client Relationships (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Net Book Value |  |
Net book value balance at the end of the period | $ 1,778.4 us-gaap_IntangibleAssetsNetExcludingGoodwill | $ 1,460.7 us-gaap_IntangibleAssetsNetExcludingGoodwill
Acquired Client Relationships |  |
Definite-lived |  |
Gross book value balance at the beginning of the period | 1,039.5 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 1,109.6 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Accumulated amortization balance at the beginning of the period | (442.8)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | (383.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net book value balance at the beginning of the period | 596.7 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 726.1 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
New Investments | 220.3 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Amortization and impairments | (122.2)us-gaap_ImpairmentOfIntangibleAssetsFinitelived / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | (128.2)us-gaap_ImpairmentOfIntangibleAssetsFinitelived / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Foreign currency translation | (4.7)us-gaap_FiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | (1.2)us-gaap_FiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Transfers and other | 0 amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | (68.9)amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net book value balance at the end of the period | 690.1 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 596.7 us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Accumulated amortization, transfers and other | 0 amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 68.9 amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net book value, transfers and other | 0 amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Gross book value balance at the end of the period | 1,255.1 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 1,039.5 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Accumulated amortization balance at the end of the period | (565.0)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | (442.8)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Indefinite-lived |  |
Net book value balance at the beginning of the period | 864.0 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 859.4 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
New investments | 244.7 us-gaap_IndefinitelivedIntangibleAssetsAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 us-gaap_IndefinitelivedIntangibleAssetsAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Amortization and Impairment | 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Foreign currency translation | (20.4)us-gaap_IndefiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 4.6 us-gaap_IndefiniteLivedIntangibleAssetsTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Transfers and other | 0 amg_IndefiniteLivedIntangibleAssetsTransferToFiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 amg_IndefiniteLivedIntangibleAssetsTransferToFiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net book value balance at the end of the period | 1,088.3 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 864.0 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net Book Value |  |
Net book value balance at the beginning of the period | 1,460.7 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 1,585.5 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
New investments | 465.0 amg_IntangibleAssetsNetExcludingGoodwillAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 amg_IntangibleAssetsNetExcludingGoodwillAcquired / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Amortization and Impairment | 122.2 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 128.2 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Foreign currency translation | (25.1)amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 3.4 amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Total, transfers and other | 0 amg_IntangibleAssetsNetExcludingGoodwillTransfersandOther / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 0 amg_IntangibleAssetsNetExcludingGoodwillTransfersandOther / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Net book value balance at the end of the period | $ 1,778.4 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | $ 1,460.7 us-gaap_IntangibleAssetsNetExcludingGoodwill / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember

X
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