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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 $ 1,778.4us-gaap_IntangibleAssetsNetExcludingGoodwill $ 1,460.7us-gaap_IntangibleAssetsNetExcludingGoodwill
Acquired Client Relationships    
Definite-lived    
Gross book value balance at the beginning of the period 1,039.5us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
1,109.6us-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 596.7us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
726.1us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
New Investments 220.3us-gaap_FinitelivedIntangibleAssetsAcquired1
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0us-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 0amg_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 690.1us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
596.7us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Accumulated amortization, transfers and other 0amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
68.9amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Net book value, transfers and other 0amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0amg_FiniteLivedIntangibleAssetsTransferAndOtherFromIndefiniteLivedIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Gross book value balance at the end of the period 1,255.1us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
1,039.5us-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 864.0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
859.4us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
New investments 244.7us-gaap_IndefinitelivedIntangibleAssetsAcquired
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0us-gaap_IndefinitelivedIntangibleAssetsAcquired
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Amortization and Impairment 0us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Foreign currency translation (20.4)us-gaap_IndefiniteLivedIntangibleAssetsTranslationAdjustments
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
4.6us-gaap_IndefiniteLivedIntangibleAssetsTranslationAdjustments
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Transfers and other 0amg_IndefiniteLivedIntangibleAssetsTransferToFiniteLivedIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0amg_IndefiniteLivedIntangibleAssetsTransferToFiniteLivedIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Net book value balance at the end of the period 1,088.3us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
864.0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Net Book Value    
Net book value balance at the beginning of the period 1,460.7us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
1,585.5us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
New investments 465.0amg_IntangibleAssetsNetExcludingGoodwillAcquired
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0amg_IntangibleAssetsNetExcludingGoodwillAcquired
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Amortization and Impairment 122.2us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
128.2us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Foreign currency translation (25.1)amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
3.4amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Total, transfers and other 0amg_IntangibleAssetsNetExcludingGoodwillTransfersandOther
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
0amg_IntangibleAssetsNetExcludingGoodwillTransfersandOther
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Net book value balance at the end of the period $ 1,778.4us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
$ 1,460.7us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember