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Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale $ 29,558.5us-gaap_AvailableForSaleSecurities $ 28,392.8us-gaap_AvailableForSaleSecurities
Trading Account 4.7us-gaap_TradingSecurities 1.7us-gaap_TradingSecurities
Contingent Consideration   55.4us-gaap_BusinessCombinationContingentConsiderationLiability
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Total Available for Sale and Trading Securities 29,563.2us-gaap_InvestmentsFairValueDisclosure
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28,394.5us-gaap_InvestmentsFairValueDisclosure
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Contingent Consideration   55.4us-gaap_BusinessCombinationContingentConsiderationLiability
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Fair Value, Measurements, Recurring | Securities Investment    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 29,558.5us-gaap_AvailableForSaleSecurities
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28,392.8us-gaap_AvailableForSaleSecurities
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Trading Account 4.7us-gaap_TradingSecurities
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1.7us-gaap_TradingSecurities
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Fair Value, Measurements, Recurring | Securities Investment | U.S. Government    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 4,506.9us-gaap_AvailableForSaleSecurities
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1,917.9us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Obligations of States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 4.6us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | U.S. Government Sponsored Agency    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 16,389.2us-gaap_AvailableForSaleSecurities
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17,528.0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Non-U.S. Government    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 310.4us-gaap_AvailableForSaleSecurities
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310.6us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Corporate Debt    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 3,577.7us-gaap_AvailableForSaleSecurities
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3,524.5us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Covered Bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 1,907.5us-gaap_AvailableForSaleSecurities
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1,943.9us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Supranational and Non-U.S. Agency Bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 360.6us-gaap_AvailableForSaleSecurities
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410.0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Residential Mortgage-Backed    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 6.4us-gaap_AvailableForSaleSecurities
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48.1us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Other Asset-Backed    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 2,321.3us-gaap_AvailableForSaleSecurities
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2,391.8us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Auction Rate Securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 18.1us-gaap_AvailableForSaleSecurities
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98.9us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Securities Investment | Others    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 155.8us-gaap_AvailableForSaleSecurities
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214.5us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Other Assets    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Assets 2,250.4ntrs_DerivativeAssetPositionFairValue
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1,734.6ntrs_DerivativeAssetPositionFairValue
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Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Assets 4,275.2ntrs_DerivativeAssetPositionFairValue
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2,865.7ntrs_DerivativeAssetPositionFairValue
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Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Assets 232.3ntrs_DerivativeAssetPositionFairValue
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Fair Value, Measurements, Recurring | Other Assets | Interest Rate Swaps    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Assets   237.9ntrs_DerivativeAssetPositionFairValue
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Fair Value, Measurements, Recurring | Other Liabilities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Liabilities 1,054.0us-gaap_DerivativeLiabilities
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1,174.9us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Liabilities 4,095.5us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Liabilities 131.8us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Swaps    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Liabilities   195.2us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Assets    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Assets (2,257.1)ntrs_DerivativeAssetPositionFairValue
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[2]
Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Liabilities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Derivative Liabilities (3,173.3)us-gaap_DerivativeLiabilities
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[2]
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Total Available for Sale and Trading Securities 4,506.9us-gaap_InvestmentsFairValueDisclosure
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 4,506.9us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment | U.S. Government    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 4,506.9us-gaap_AvailableForSaleSecurities
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | U.S. Government Sponsored Agency    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 16,389.2us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Non-U.S. Government    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 310.4us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Corporate Debt    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 3,577.7us-gaap_AvailableForSaleSecurities
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3,524.5us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Covered Bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 1,907.5us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational and Non-U.S. Agency Bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 360.6us-gaap_AvailableForSaleSecurities
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410.0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Residential Mortgage-Backed    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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48.1us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Other Asset-Backed    
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2,391.8us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Others    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Available for Sale 155.8us-gaap_AvailableForSaleSecurities
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214.5us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Interest Rate Contracts    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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[1] Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2014, derivative assets and liabilities shown above also include reductions of $315.8 million and $1,232.0 million, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
[2] Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2013, derivative assets and liabilities shown above also include reductions of $210.7 million and $767.7 million, respectively, as a result of cash collateral received from and deposited with derivative counterparties.