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Derivative Financial Instruments - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative [Line Items]      
Derivative assets recorded on the consolidated balance sheet, reduced as a result of master netting agreements $ 1,900,000,000ntrs_DerivativeAssetsReducedForMasterNettingAgreements $ 1,200,000,000ntrs_DerivativeAssetsReducedForMasterNettingAgreements  
Derivative liabilities recorded on the consolidated balance sheet, reduced as a result of master netting agreements 1,900,000,000ntrs_DerivativeLiabilitiesReducedForMasterNettingAgreements 1,200,000,000ntrs_DerivativeLiabilitiesReducedForMasterNettingAgreements  
Reduction in derivative assets due to cash collateral received from derivative counterparties 315,800,000us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral 210,700,000us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral  
Reduction in derivative liabilities due to cash collateral deposited with derivative counterparties 1,200,000,000us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral 767,700,000us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral  
This amount represents additional cash collateral received from derivative counterparties not offset against derivative assets 19,600,000ntrs_CashCollateralReceivedFromNotOffsetAgainstDerivativeAssets 36,400,000ntrs_CashCollateralReceivedFromNotOffsetAgainstDerivativeAssets  
This amount represents additional cash collateral deposited with derivative counterparties not offset against derivative liabilities 153,200,000ntrs_CashCollateralDepositedWithNotOffsetAgainstDerivativeLiabilities 39,300,000ntrs_CashCollateralDepositedWithNotOffsetAgainstDerivativeLiabilities  
Securities posted as collateral 27,400,000ntrs_SecuritiesHeldAsCollateralAtCost 27,600,000ntrs_SecuritiesHeldAsCollateralAtCost  
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position 299,500,000us-gaap_DerivativeNetLiabilityPositionAggregateFairValue 257,300,000us-gaap_DerivativeNetLiabilityPositionAggregateFairValue  
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position 272,900,000us-gaap_CollateralAlreadyPostedAggregateFairValue 197,000,000us-gaap_CollateralAlreadyPostedAggregateFairValue  
Termination payments that could have been required for derivative instruments with credit-risk-related contingent features 26,600,000ntrs_CreditRiskRelatedContingentFeaturesCollateralThreshold 60,300,000ntrs_CreditRiskRelatedContingentFeaturesCollateralThreshold  
Changes recorded in the fair value of the hedged items in a fair value hedge 0us-gaap_ChangeInUnrealizedGainLossOnHedgedItemInFairValueHedge1 (900,000)us-gaap_ChangeInUnrealizedGainLossOnHedgedItemInFairValueHedge1 400,000us-gaap_ChangeInUnrealizedGainLossOnHedgedItemInFairValueHedge1
The net gain/loss recognized in earnings during the period representing the amount of hedge ineffectiveness for fair value hedges of available for sale investment securities, senior notes, or subordinated debt 0us-gaap_GainLossOnFairValueHedgeIneffectivenessNet (800,000)us-gaap_GainLossOnFairValueHedgeIneffectivenessNet 300,000us-gaap_GainLossOnFairValueHedgeIneffectivenessNet
Maximum length of time being hedged for exposure to variability in future cash flows of forecasted foreign currency denominated transactions 23 months    
The net gain/loss recognized in earnings during the period representing the amount of hedge ineffectiveness cash flow hedges 0us-gaap_GainLossOnCashFlowHedgeIneffectivenessNet 0us-gaap_GainLossOnCashFlowHedgeIneffectivenessNet 0us-gaap_GainLossOnCashFlowHedgeIneffectivenessNet
Net foreign exchange contract gains (losses) reclassified from AOCI 0us-gaap_GainLossOnDiscontinuationOfCashFlowHedgeDueToForecastedTransactionProbableOfNotOccurringNet 0us-gaap_GainLossOnDiscontinuationOfCashFlowHedgeDueToForecastedTransactionProbableOfNotOccurringNet 200,000us-gaap_GainLossOnDiscontinuationOfCashFlowHedgeDueToForecastedTransactionProbableOfNotOccurringNet
Ineffectiveness from net investment hedges 0us-gaap_AmountOfIneffectivenessOnNetInvestmentHedges 0us-gaap_AmountOfIneffectivenessOnNetInvestmentHedges 5,300,000us-gaap_AmountOfIneffectivenessOnNetInvestmentHedges
Foreign Currency Denominate      
Derivative [Line Items]      
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges (5,700,000)us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
/ us-gaap_InformationByFinancialStatementLineItemAxis
= ntrs_ForeignCurrencyDenominateMember
   
Available for Sale Investment Securities      
Derivative [Line Items]      
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges 3,700,000us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
/ us-gaap_InformationByFinancialStatementLineItemAxis
= us-gaap_AvailableforsaleSecuritiesMember
   
Net Investment Hedges      
Derivative [Line Items]      
Sterling denominated debt designated as hedges of foreign exchange risk $ 243,900,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember
$ 259,100,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_NetInvestmentHedgingMember
 
Client Related and Trading      
Derivative [Line Items]      
Percentage of derivatives outstanding related to client-related and trading activities 97.00%ntrs_PercentageOfDerivativesOutstanding
/ us-gaap_FinancialInstrumentAxis
= ntrs_ClientRelatedAndTradingMember
97.00%ntrs_PercentageOfDerivativesOutstanding
/ us-gaap_FinancialInstrumentAxis
= ntrs_ClientRelatedAndTradingMember