Represents percentage of assumed growth rates, used as an input to measure fair value.

No definition available.

Name:	amg_FairValueInputsGrowthRates
Namespace Prefix:	amg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

+ Details

Name:	us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

Fair Value Measurements - Schedule of Quantitative Information used in Valuing Level 3 Liabilities (Details) (USD $) In Millions, unless otherwise specified					9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Discounted Cash Flow Contingent Payment Arrangements	Dec. 31, 2013 Discounted Cash Flow Contingent Payment Arrangements	Sep. 30, 2014 Discounted Cash Flow Contingent Payment Arrangements Minimum	Dec. 31, 2013 Discounted Cash Flow Contingent Payment Arrangements Minimum	Sep. 30, 2014 Discounted Cash Flow Contingent Payment Arrangements Maximum	Dec. 31, 2013 Discounted Cash Flow Contingent Payment Arrangements Maximum	Sep. 30, 2014 Discounted Cash Flow Affiliate Equity Repurchase Obligations	Dec. 31, 2013 Discounted Cash Flow Affiliate Equity Repurchase Obligations	Sep. 30, 2014 Discounted Cash Flow Affiliate Equity Repurchase Obligations Minimum	Dec. 31, 2013 Discounted Cash Flow Affiliate Equity Repurchase Obligations Minimum	Sep. 30, 2014 Discounted Cash Flow Affiliate Equity Repurchase Obligations Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities			$ 57.0	$ 50.2					$ 20.5	$ 4.0
Growth rate (as a percent)					3.00%	3.00%	9.00%	11.00%			3.00%	8.00%	10.00%
Discount rate (as a percent)					12.00%	14.00%	17.00%	18.00%			15.00%	15.00%	16.00%
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0