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Equity Distribution Program (Details) - Equity Distribution Program - USD ($)	9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019
Class of Stock [Line Items]
Equity distribution program, maximum amount authorized		$ 500,000,000.0
Sales of common stock under the equity distribution program	$ 0

X

- Definition

Stock Issuable, Value, Maximum

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

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

X

- Definition

The cash inflow from the additional capital contribution to the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

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- Details

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