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Benefit Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) plan	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Compensation and Retirement Disclosure [Abstract]
Number of defined contribution plans | plan	3
Consolidated expenses related to benefit plans | $	$ 18.7	$ 17.2	$ 14.1

X

- Definition

The number of defined contribution plans sponsored by the entity.

+ References

No definition available.

+ Details

Name:	amg_DefinedContributionPlansNumber
Namespace Prefix:	amg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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