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Afilliate Equity - Schedule of the Effect of Changes in the Company's Ownership Interest in its Affiliates on the Controlling Interest's Equity (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Affiliate Equity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (controlling interest) | $ 172.6 us-gaap_NetIncomeLoss | $ 103.2 us-gaap_NetIncomeLoss | [1] | $ 99.1 us-gaap_NetIncomeLoss | [1] | $ 77.2 us-gaap_NetIncomeLoss | $ 158.2 us-gaap_NetIncomeLoss | $ 75.2 us-gaap_NetIncomeLoss | $ 64.7 us-gaap_NetIncomeLoss | $ 62.4 us-gaap_NetIncomeLoss | $ 452.1 us-gaap_NetIncomeLoss | $ 360.5 us-gaap_NetIncomeLoss | $ 174.0 us-gaap_NetIncomeLoss
Increase (decrease) in controlling interest paid-in capital from the sale of Affiliate equity |  |  |  |  |  |  |  |  |  |  | (33.3)amg_AdjustmentsToAdditionalPaidInCapitalIncreaseDecreaseInCarryingAmountOfRedeemableNoncontrollingInterests | (74.0)amg_AdjustmentsToAdditionalPaidInCapitalIncreaseDecreaseInCarryingAmountOfRedeemableNoncontrollingInterests | (44.0)amg_AdjustmentsToAdditionalPaidInCapitalIncreaseDecreaseInCarryingAmountOfRedeemableNoncontrollingInterests
Change from Net income (controlling interest) and net transfers with non-controlling interests |  |  |  |  |  |  |  |  |  |  | $ 418.8 amg_IncreaseDecreaseInEquityDueToProfitLossAndNetTransfersWithNoncontrollingInterests | $ 286.5 amg_IncreaseDecreaseInEquityDueToProfitLossAndNetTransfersWithNoncontrollingInterests | $ 130.0 amg_IncreaseDecreaseInEquityDueToProfitLossAndNetTransfersWithNoncontrollingInterests
[1] | In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

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- Definition The increase (decrease) in additional paid-in capital of the reporting entity from purchases and sales of Affiliate equity. + References No definition available. + Details
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- Definition Increase (decrease) in equity during the period due to net income attributable to the parent company and net transfers with non-controlling interests. + References No definition available. + Details
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- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Other Comprehensive Income -URI http://asc.fasb.org/extlink&oid=6519514 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Net Income -URI http://asc.fasb.org/extlink&oid=6518256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.19) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.18) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.22) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 5 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 20 -Article 9 + Details
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