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Compensation Expense for Share-Based Payment Arrangements and Associated Tax Impacts (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-Based Compensation Expense $ 22.9us-gaap_AllocatedShareBasedCompensationExpense $ 23.1us-gaap_AllocatedShareBasedCompensationExpense
Tax Benefits Recognized 8.6us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 8.7us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Restricted Stock Unit Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-Based Compensation Expense 13.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 14.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-Based Compensation Expense 6.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 6.4us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Performance Stock Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-Based Compensation Expense $ 3.4us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember		 $ 2.6us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= ntrs_PerformanceStockUnitsMember