Debt Instrument, Individual Security, Share Price

No definition available.

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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the deferred taxes generated from the excess of the interest expense calculated under federal income tax regulations over interest expense as calculated for book purposes.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031898-161870

+ Details

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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Price of the entity's common stock which would be required to be attained for the conversion feature embedded in the debt instrument to become effective.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold period of specified consecutive trading days within which common stock price to conversion price of convertible debt instrument must exceed threshold percentage for specified number of trading days to trigger conversion feature.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold number of specified trading days that common stock price to conversion price of convertible debt instruments must exceed threshold percentage within a specified consecutive trading period to trigger conversion feature.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Incremental percentage increase (decrease) in the stated rate on a debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentInterestRateIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeeAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. 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/disclosureRef
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=amg_SeniorUnsecuredCreditFacilityRevolverMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=amg_SeniorUnsecuredTermLoanMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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- Details

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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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Balance Type:
Period Type:

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Debt - Additional Information (Details)		1 Months Ended	12 Months Ended
Debt - Additional Information (Details)	Jan. 08, 2021	Aug. 31, 2019 $ / shares	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Debt, par value, maturing in 2024			$ 400,000,000.0
Debt, par value, maturing in 2025			350,000,000.0
Term loan outstanding			2,312,100,000	$ 1,793,800,000
Senior bank debt
Debt Instrument [Line Items]
Term loan outstanding			349,800,000	449,700,000
Senior bank debt \| Revolver
Debt Instrument [Line Items]
Maximum borrowing capacity			1,250,000,000
Maximum borrowing capacity, additional amount			500,000,000.0
Outstanding borrowings			0	0
Commitment fee amount			1,500,000	1,500,000
Senior bank debt \| Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity			350,000,000.0
Maximum borrowing capacity, additional amount			75,000,000.0
Term loan outstanding			$ 350,000,000.0	$ 450,000,000.0
Weighted average interest rate on amount outstanding			1.02%	2.66%
Senior bank debt \| Term Loan \| LIBOR
Debt Instrument [Line Items]
Basis spread over variable debt rate			0.875%
Senior bank debt \| Term Loan \| LIBOR \| Subsequent Event
Debt Instrument [Line Items]
Basis spread over variable debt rate	0.95%
Increase to marginal rate	0.075%
Senior notes
Debt Instrument [Line Items]
Term loan outstanding			$ 1,091,900,000	$ 743,800,000
Senior notes \| 2024 Senior Notes
Debt Instrument [Line Items]
Effective interest rate			4.42%
Stated interest rate			4.25%
Senior notes \| 2025 Senior Notes
Debt Instrument [Line Items]
Effective interest rate			3.66%
Stated interest rate			3.50%
Senior notes \| 2030 Senior Notes
Debt Instrument [Line Items]
Effective interest rate			3.39%
Stated interest rate			3.30%
Junior subordinated notes
Debt Instrument [Line Items]
Term loan outstanding			$ 556,400,000	289,700,000
Junior subordinated notes \| 2059 Junior Subordinated Notes
Debt Instrument [Line Items]
Effective interest rate			5.90%
Stated interest rate			5.875%
Junior subordinated notes \| 2060 Junior Subordinated Notes
Debt Instrument [Line Items]
Effective interest rate			4.83%
Stated interest rate			4.75%
Junior convertible securities
Debt Instrument [Line Items]
Term loan outstanding			$ 314,000,000.0	$ 310,600,000
Stated interest rate			5.15%
Deferred tax liability			$ 9,000,000
Conversion ratio (in shares)		0.2558
Principal amount at maturity (in dollars per share) \| $ / shares		$ 50.00
Conversion price (in dollars per share) \| $ / shares		$ 195.47
Redemption closing price trigger (in dollars per share) \| $ / shares			$ 254.10
Number of trading days closing price has exceeded threshold			20
Number of consecutive trading days			30