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Changes in Level 3 Liabilities (Detail) (Contingent Consideration, USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Contingent Consideration
   
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair Value Liabilities Beginning Balance   $ 55.4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
Total (Gains) Losses:    
Included in Earnings   (0.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
[1]
Included in Other Comprehensive Income 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
Purchases, Issues, Sales, and Settlements    
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
Settlements   (55.3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at March 31 $ 0ntrs_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesChangeInUnrealizedGainsLossesIncludedInEarningsOnInstrumentsStillHeld
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
[1] $ 0ntrs_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesChangeInUnrealizedGainsLossesIncludedInEarningsOnInstrumentsStillHeld
/ us-gaap_FairValueByLiabilityClassAxis
= ntrs_ContingentConsiderationMember
[1]
[1] (Gains) losses are recorded in other operating income (expense) in the consolidated statements of income.