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v2.4.1.9

Senior Notes - Summary of Principle Terms of Senior Notes (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Debt Instrument |  |
Par value (in millions) | $ 430.8 us-gaap_DebtInstrumentFaceAmount | $ 730.8 us-gaap_DebtInstrumentFaceAmount
2022 Senior Notes |  |
Debt Instrument |  |
Par value (in millions) | 140.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2022Member |
Stated coupon | 5.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2022Member |
2024 Senior Notes |  |
Debt Instrument |  |
Par value (in millions) | 400.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2024Member |
Stated coupon | 4.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2024Member |
2042 Senior Notes |  |
Debt Instrument |  |
Par value (in millions) | $ 200.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesSixPointThreeSevenFivePercentDue2042Member |
Stated coupon | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesSixPointThreeSevenFivePercentDue2042Member |

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Details
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