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Investments in Marketable Securities - Summary of the Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments (Details) (Equity Securities, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Equity Securities
Available-for-Sale
Cost	$ 125.6us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	$ 103.2us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Unrealized Gains	42.8amg_AvailableforsaleSecuritiesGrossUnrealizedGain1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	33.3amg_AvailableforsaleSecuritiesGrossUnrealizedGain1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Unrealized Losses	(18.1)amg_AvailableforsaleSecuritiesGrossUnrealizedLoss1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	(1.1)amg_AvailableforsaleSecuritiesGrossUnrealizedLoss1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Fair Value	150.3us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	135.4us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Trading
Cost	19.5us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	17.9us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Unrealized Gains	2.9us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	4.6us-gaap_TradingSecuritiesUnrealizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Unrealized Losses	(0.1)us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	0us-gaap_TradingSecuritiesUnrealizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Fair Value	$ 22.3us-gaap_TradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember	$ 22.5us-gaap_TradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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Available-for-sale Securities, Gross Unrealized Gain1

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities.

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Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities.

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