Debt - Senior Bank Debt (Details) - Senior bank debt - USD ($)	6 Months Ended
Jun. 30, 2023	Dec. 31, 2022
Revolver
Debt Instrument [Line Items]
Available borrowing	$ 1,250,000,000
Additional borrowing amount	500,000,000
Borrowings outstanding	0	$ 0
Term Loan
Debt Instrument [Line Items]
Available borrowing	350,000,000
Additional borrowing amount	$ 75,000,000
Term Loan \| Debt Instrument, Adjustment Rate
Debt Instrument [Line Items]
Basis spread on debt variable rate	0.10%
Term Loan \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on debt variable rate	0.85%

Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount

No definition available.

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Namespace Prefix:	amg_
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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- 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.

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+ Details

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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.

+ References

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+ Details

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Balance Type:	credit
Period Type:	instant

- 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

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Balance Type:	credit
Period Type:	instant

- Details

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