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Senior Bank Debt (Details) (USD $)	12 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Apr. 15, 2014
Debt Instrument
Principal amount at maturity	$ 430,800,000us-gaap_DebtInstrumentFaceAmount	$ 730,800,000us-gaap_DebtInstrumentFaceAmount
Term loan outstanding	855,000,000us-gaap_LongTermDebt	525,000,000us-gaap_LongTermDebt
Senior Unsecured Revolving Credit Facility
Debt Instrument
Maximum borrowing capacity	1,250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember
Line of credit facility amount outstanding	605,000,000us-gaap_LineOfCredit / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember	525,000,000us-gaap_LineOfCredit / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember
Weighted average interest rate	1.40%us-gaap_DebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember	1.95%us-gaap_DebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember
Commitment fee amount	2,800,000us-gaap_LineOfCreditFacilityCommitmentFeeAmount / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember	3,200,000us-gaap_LineOfCreditFacilityCommitmentFeeAmount / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredCreditFacilityMember
Senior Unsecured Term Loan
Debt Instrument
Principal amount at maturity			250,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredTermLoanMember
Debt instrument term			5 years
Maximum borrowing capacity, additional amount			100,000,000amg_DebtInstrumentMaximumBorrowingCapacityAdditionalAmount / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredTermLoanMember
Term loan outstanding	$ 250,000,000us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredTermLoanMember
Weighted average interest rate on amount outstanding	1.48%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = amg_SeniorUnsecuredTermLoanMember

X

- Definition

Debt Instrument, Maximum Borrowing Capacity, Additional Amount

+ References

No definition available.

+ Details

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Balance Type:	credit
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X

- 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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- Details

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X

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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X

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Period Type:	instant

X

- Definition

Amount of the fee for available but unused credit capacity under the credit facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Number 210

-Section 02

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X

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7



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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



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-Publisher SEC

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X

- Definition

Weighted average interest rate of long-term debt outstanding.

+ References

No definition available.

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