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Subsequent Events (Details)		6 Months Ended
	Jul. 13, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jul. 13, 2018 GBP (£)
Subsequent Event [Line Items]
Repayments of long-term debt		$ 540,000,000	$ 285,000,000
Unsecured Debt | Term Loan
Subsequent Event [Line Items]
Principal amount at maturity		385,000,000.0
Maximum borrowing capacity, additional amount		$ 65,000,000.0
Unsecured Debt | Term Loan | Subsequent Event
Subsequent Event [Line Items]
Principal amount at maturity	$ 385,000,000			£ 290,000,000
Repayments of long-term debt	1,500,000
Maximum borrowing capacity, additional amount	$ 65,000,000.0			£ 50,000,000.0

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- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
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Balance Type:	credit
Period Type:	instant

X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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X

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

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X

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

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- Details

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- Details

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