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Share-Based Compensation Plans - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost related to unvested share-based compensation $ 112.6us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Weighted average period of recognition of unvested share-based compensation (in years) 3 years    
Share-Based Compensation Expense 77.7us-gaap_AllocatedShareBasedCompensationExpense 73.8us-gaap_AllocatedShareBasedCompensationExpense 73.9us-gaap_AllocatedShareBasedCompensationExpense
Stock options under the Plan, the 2002 Plan, and the 1992 Plan | Upper Limit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Option maximum term (in years) 10 years    
Options vest and become exercisable after the date of grant (in years) 4 years    
Stock options under the Plan, the 2002 Plan, and the 1992 Plan | Lower Limit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of the fair value for options to purchase common stock on the date the options are granted 100.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate
/ us-gaap_AwardTypeAxis
= ntrs_StockOptionPlansMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Options vest and become exercisable after the date of grant (in years) 1 year    
Performance Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted stock units granted 249,618us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
296,650us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
 
Restricted stock units weighted average grant date fair value $ 53.08us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
$ 49.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
 
performance based units vesting period 3 years    
Aggregate intrinsic value of options granted 48.8ntrs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
30.6ntrs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
 
Vesting term of units granted 2 years 2 months 12 days 2 years 8 months 12 days  
Share-Based Compensation Expense 12.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
7.4us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
2.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
Performance Stock Units | Upper Limit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage at which units can vest 125.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Performance Stock Units | Lower Limit      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage at which units can vest 0.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
2012 Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares of common stock authorized for issuance under the Plan 30,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= ntrs_StockIncentivePlanTwentyTwelveMember
   
Shares available for future grant under the Plan 29,803,955us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= ntrs_StockIncentivePlanTwentyTwelveMember
   
Non Employee Directors, Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total value of stock awards granted 1.0us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionStockholdersEquity
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
1.1us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionStockholdersEquity
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
0.9us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionStockholdersEquity
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
Number of stock units granted 16,770us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
20,599us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
20,148us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
Share-Based Compensation Expense 1.0us-gaap_AllocatedShareBasedCompensationExpense
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
1.1us-gaap_AllocatedShareBasedCompensationExpense
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
0.9us-gaap_AllocatedShareBasedCompensationExpense
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= ntrs_NonEmployeeDirectorsPlanMember
Restricted Stock Unit Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock units vesting description The restricted stock units granted in 2014 predominately vest at a rate equal to 50% on the third anniversary date of the grant and 50% on the fourth anniversary date.    
Number of restricted stock units granted 1,086,241us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
1,181,321us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
988,421us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
Restricted stock units weighted average grant date fair value $ 61.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 52.82us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 43.72us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
Fair value of restricted stock units vested $ 63.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 47.0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 21.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
Restricted Stock Unit Awards | Third Anniversary Of Grant Date      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage at which units can vest 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ ntrs_PeriodAxis
= ntrs_ThirdAnniversaryOfGrantDateMember
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Restricted Stock Unit Awards | Fourth Anniversary Of Grant Date      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage at which units can vest 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ ntrs_PeriodAxis
= ntrs_FourthAnniversaryOfGrantDateMember
/ ntrs_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPlanNameAxis
= us-gaap_RestrictedStockUnitsRSUMember