XML 56 R42.htm IDEA: XBRL DOCUMENT v3.10.0.1

Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Marketable Securities [Abstract]
Cost	$ 128.3	$ 67.1
Unrealized gains	15.8	13.0
Unrealized losses	0.0	(2.3)
Fair value	$ 144.1	$ 77.8

X

- Definition

Marketable Securities, Cost

+ References

No definition available.

+ Details

Name:	amg_MarketableSecuritiesCost
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Marketable Securities, Unrealized Gains

+ References

No definition available.

+ Details

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

X

- Definition

Marketable Securities, Unrealized Losses

+ References

No definition available.

+ Details

Name:	amg_MarketableSecuritiesUnrealizedLosses
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

X

- References

No definition available.

+ Details

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