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Senior Notes - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) note
Debt Instrument
Number of senior notes outstanding | note	2
2022 Senior Notes
Debt Instrument
Notes redeemed, canceled and retired | $	$ 200,000,000.0
Stated interest rate	6.375%
Redemption price percentage	100.00%

X

- Definition

Represents the number of debt instruments outstanding during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
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Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	instant

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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.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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Period Type:	duration

X

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

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

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- Details

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Namespace Prefix:
Data Type:	na
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Period Type: