Tax deduction period for goodwill and client relationships amortization expense.

No definition available.

Name:	amg_AcquiredGoodwillAndClientRelationshipsAmortizationPeriodForTaxDeductions
Namespace Prefix:	amg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum amount of payments to be made by the Company for potential payments under the contingent consideration arrangement including cash and shares.

+ References

No definition available.

+ Details

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

- Definition

Represents projected payments, as of the acquisition date, of potential payments under the contingent consideration arrangement including cash and shares.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of Consideration Paid, Deferred Consideration, noncontrolling interest and Contingent payment obligations related to acquisition.

+ References

No definition available.

+ Details

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

- Definition

Percentage of goodwill, attributable to business segment, arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed.

+ References

No definition available.

+ Details

Name:	amg_BusinessAcquisitionPurchasePriceAllocationGoodwillPercentage
Namespace Prefix:	amg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the maximum period in which amount, measured at acquisition-date fair value, of all liabilities assumed that arise from contingencies and will be recognized by the entity.

+ References

No definition available.

+ Details

Name:	amg_BusinessCombinationLiabilitiesArisingFromContingenciesAmountToBeRecognizedAfterPeriodOfAcquisitionMaximum
Namespace Prefix:	amg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum period in which amount, measured at acquisition-date fair value, of all liabilities assumed that arise from contingencies and will be recognized by the entity.

+ References

No definition available.

+ Details

Name:	amg_BusinessCombinationLiabilitiesArisingFromContingenciesAmountToBeRecognizedAfterPeriodOfAcquisitionMinimum
Namespace Prefix:	amg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash paid to acquire the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph f(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Details

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

- Definition

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=7659399&loc=d3e1392-128463

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 54

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 55

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph r(2, 3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Definition

The amount of acquisition cost of a business combination allocated to an identifiable intangible asset that will be amortized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph e

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 52

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

The total purchase price of the acquired entity. This includes cash paid to equity interest holders of the acquired entity, fair value of debt and equity securities issued to equity holders of the acquired entity, and transaction costs paid to third parties to consummate the acquisition.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 35

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

Amount of goodwill arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 52

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 53

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

The amount of acquisition cost of a business combination allocated to tangible assets.

+ References

No definition available.

+ Details

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

- Definition

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the fair value of the noncontrolling interest in the acquiree at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)(1)

-URI http://asc.fasb.org/extlink&oid=6910749&loc=d3e4845-128472

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph p(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount, measured at acquisition-date fair value, of all liabilities assumed that arise from contingencies and were recognized by the entity.

+ References

+ Details

Name:	us-gaap_BusinessCombinationLiabilitiesArisingFromContingenciesAmountRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

+ References

+ Details

Name:	us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

+ References

+ Details

Name:	us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Business Combinations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended		0 Months Ended		0 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Jun. 29, 2012 Veritable, LP	Jun. 29, 2012 Veritable, LP High Net Worth	Jun. 29, 2012 Yacktman Asset Management Co	Sep. 30, 2012 Yacktman Asset Management Co	Jun. 29, 2012 Yacktman Asset Management Co Mutual Fund	Jun. 29, 2012 Yacktman Asset Management Co High Net Worth
Business Combinations
Goodwill percentage attributable to segments				100.00%			91.00%	9.00%
Tax deduction period for goodwill and client relationships amortization expense			15 years		15 years
Purchase price allocations
Consideration paid			$ 116.8		$ 301.0
Non-controlling interests			30.8		213.3
Contingent payment arrangement					24.8	24.8
Enterprise value			147.6		539.1
Acquired client relationships			85.1		367.5
Tangible assets, net			2.7		9.0
Goodwill			59.8		162.6
Total			147.6		539.1
Maximum contingent payments to be made					75.0
Contingencies amount to be recognized after period of acquisition, minimum					3 years
Contingencies amount to be recognized after period of acquisition, maximum					5 years
Projected amount of the contingent payment obligation					52.6
Unaudited pro forma financial results
Revenue	1,390.0	1,378.3
Net Income (controlling interest)	111.0	134.8
Earnings per share - basic (in dollars per share)	$ 2.15	$ 2.60
Earnings per share - diluted (in dollars per share)	$ 2.10	$ 2.53
New Affiliate investments contribution
New Affiliate investments contributed revenue	43.2
New Affiliate investments contributed earnings	$ 7.0