Fair Value Assumptions, Average Projected Growth Rate

No definition available.

Name:	amg_FairValueAssumptionsAverageProjectedGrowthRate
Namespace Prefix:	amg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Assumptions, Average Projected Growth Rate Period

+ References

No definition available.

+ Details

Name:	amg_FairValueAssumptionsAverageProjectedGrowthRatePeriod
Namespace Prefix:	amg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Assumptions, Discount Rate For Assets Based Fees

+ References

No definition available.

+ Details

Name:	amg_FairValueAssumptionsDiscountRateForAssetsBasedFees
Namespace Prefix:	amg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Assumptions, Discount Rate For Performance Fees

+ References

No definition available.

+ Details

Name:	amg_FairValueAssumptionsDiscountRateForPerformanceFees
Namespace Prefix:	amg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Assumptions, Market Participant Tax Rate

+ References

No definition available.

+ Details

Name:	amg_FairValueAssumptionsMarketParticipantTaxRate
Namespace Prefix:	amg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

+ Details

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

- Definition

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 35
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=124268079&loc=d3e32787-111569

+ Details

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

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in next 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in remainder of current fiscal year.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized 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).

+ References

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized 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

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in third 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

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in second 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

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsNetAmortizationExpenseRollingMaturityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in next 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

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in remainder of current fiscal year.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitmentsFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Name:	us-gaap_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_EquityMethodInvesteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Equity Method Investments in Affiliates - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Acquired Client Relationships
Schedule of Equity Method Investments [Line Items]
Amortization of Intangible Assets				$ 25.3
Estimated Future Amortization Expense of Finite-Lived Intangibles
Finite-lived intangible, future amortization expense, remainder of fiscal year	$ 9.0			9.0
Finite-lived intangible, future amortization expense in year one	30.0			30.0
Finite-lived intangible, future amortization expense in year two	30.0			30.0
Finite-lived intangible, future amortization expense in year three	15.0			15.0
Finite-lived intangible, future amortization expense in year four	10.0			10.0
Finite-lived intangible, future amortization expense in year five	10.0			10.0
Equity Method Investee
Estimated Future Amortization Expense of Finite-Lived Intangibles
Impairment expense			$ 140.0
Average projected growth rate period			5 years
Average projected growth rate			(2.00%)
Discount rate for assets based fees			11.00%
Discount rate for performance fees			20.00%
Market participant tax rate			25.00%
Total contingent payments	103.9			103.9
Contingent payments in remainder of fiscal year	63.9			63.9
Contingent payments in year one	40.0			40.0
Equity Method Investee \| Acquired Client Relationships
Schedule of Equity Method Investments [Line Items]
Amortization of Intangible Assets	29.3	$ 34.3		93.8	$ 110.5
Estimated Future Amortization Expense of Finite-Lived Intangibles
Finite-lived intangible, future amortization expense, remainder of fiscal year	30.0			30.0
Finite-lived intangible, future amortization expense in year one	80.0			80.0
Finite-lived intangible, future amortization expense in year two	80.0			80.0
Finite-lived intangible, future amortization expense in year three	50.0			50.0
Finite-lived intangible, future amortization expense in year four	50.0			50.0
Finite-lived intangible, future amortization expense in year five	$ 50.0			$ 50.0