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Income Taxes Income Taxes - Valuation Allowance (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	$ (6.5)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount	$ (1.6)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount	$ (11.5)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Operating Loss Carryforwards
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	(15.7)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = us-gaap_ValuationAllowanceOperatingLossCarryforwardsMember
Increase in State Uncertain Tax Positions
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	$ 9.2us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = amg_IncreaseinStateUncertainTaxPositionsMember

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- Definition

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 45

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32123-109318

+ Details

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