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Loans and Leases - Additional Information (Detail) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Loan
Dec. 31, 2013
Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Collateral required on Residential real estate loan, lower limit 65.00%ntrs_LowerLimitOfGenerallyRequiredLoanToCollateralValueForResidentialRealEstateLoans      
Collateral required on Residential real estate loan, upper limit 80.00%ntrs_UpperLimitOfGenerallyRequiredLoanToCollateralValueForResidentialRealEstateLoans      
Draw period for credit line products 10 years      
Other U.S. loans and non-U.S. loans, short duration advances $ 1,700,000,000ntrs_LoansReceivableOverdraftAdvances   1,500,000,000ntrs_LoansReceivableOverdraftAdvances  
Demand deposits 77,600,000ntrs_OverdrawnDemandDepositAccounts   92,100,000ntrs_OverdrawnDemandDepositAccounts  
Loans held for sale 0us-gaap_LoansHeldForSaleMortgages   2,500,000us-gaap_LoansHeldForSaleMortgages  
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures 250,000ntrs_SmallerHomogeneousLoanThreshold      
Interest income that would have been recorded for nonperforming loans 2,100,000ntrs_LoansAndLeasesReceivableNonPerformingLoansInterestThatWouldHaveBeenRecognized 2,500,000ntrs_LoansAndLeasesReceivableNonPerformingLoansInterestThatWouldHaveBeenRecognized    
Undrawn loan commitments and standby letters of credit 2,300,000ntrs_LoansAndLeasesReceivableCommitmentToLend   2,400,000ntrs_LoansAndLeasesReceivableCommitmentToLend  
Total Loans and Leases 32,630,200,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome   31,640,200,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome  
Unpaid principal balance 301,700,000us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance   301,000,000us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance  
Foreclosed residential real estate properties with carrying value 5,700,000us-gaap_RealEstateAcquiredThroughForeclosure      
Consumer loans with carrying value collateralized by residential real estate property 29,300,000us-gaap_LoansAndLeasesReceivableConsumerRealEstate      
Lower Limit        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection 90 days      
Upper Limit        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Number of days past due for disclosure purposes to be categorized as a current 29 days      
First Lien        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans held by Northern trust 89.00%ntrs_PercentageCollateralToLoans
/ us-gaap_LongtermDebtTypeAxis
= ntrs_FirstLienMember		   89.00%ntrs_PercentageCollateralToLoans
/ us-gaap_LongtermDebtTypeAxis
= ntrs_FirstLienMember		  
Equity        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Equity credit lines 1,700,000,000us-gaap_LineOfCredit
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_EquityMember		   1,800,000,000us-gaap_LineOfCredit
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_EquityMember		  
Nonperforming Financing Receivable        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Total Loans and Leases 85,900,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_NonperformingFinancingReceivableMember		   82,700,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_NonperformingFinancingReceivableMember		  
Performing Financing Receivable        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Total Loans and Leases 63,000,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_PerformingFinancingReceivableMember		   68,600,000us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_PerformingFinancingReceivableMember		  
Performing Financing Receivable | Lower Limit        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan 6 months      
Troubled debt restructuring loans and leases receivable to Performing status 6 months      
Residential Real Estate | Re-Defaulted        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Number of Loans and Leases     4ntrs_FinancingReceivableModificationsNumberOfContractsInPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= ntrs_RedefaultedMember		 0ntrs_FinancingReceivableModificationsNumberOfContractsInPeriod
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= ntrs_RedefaultedMember
Recorded Investment 700,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= ntrs_RedefaultedMember		      
Unpaid principal balance $ 700,000us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialRealEstateMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= ntrs_RedefaultedMember