XML 23 R78.htm IDEA: XBRL DOCUMENT v2.4.1.9

Business Combinations - Schedule of Goodwill Allocation by Segment (Details)	Dec. 31, 2014
SouthernSun | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	37.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
SouthernSun | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	23.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
SouthernSun | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	40.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_SouthernSunMember / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
River Road | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	31.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
River Road | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	51.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
River Road | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	18.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_RiverRoadMember / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Veritas | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	24.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Veritas | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	76.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Veritas | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	0.00%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount / us-gaap_BusinessAcquisitionAxis = amg_VeritasMember / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember

X

- Details

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

X

- Definition

Percentage of fair value of reporting unit in excess of carrying amount.

+ References

No definition available.

+ Details

Name:	us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=amg_RiverRoadMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=amg_SouthernSunMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=amg_VeritasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: