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Off Balance Sheet Financial Instruments - Additional Information (Detail) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Other Off Balance Sheet Financial Instruments [Line Items]    
Minimum collateral maintained against fair value of client securities loaned plus accrued interest 100.00%ntrs_SecuritiesLendingRelatedCollateralMaintenance  
Estimated credit exposure from clearing activities $ 60,000,000ntrs_EstimatedCreditExposureFromClearingActivities $ 73,000,000ntrs_EstimatedCreditExposureFromClearingActivities
Indemnification Agreement    
Other Off Balance Sheet Financial Instruments [Line Items]    
Securities loaned and subject to indemnification 98,100,000,000ntrs_SecuritiesLoanedSubjectToIndemnification
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_IndemnificationGuaranteeMember		 82,700,000,000ntrs_SecuritiesLoanedSubjectToIndemnification
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_IndemnificationGuaranteeMember
Credit loss liability $ 0ntrs_CreditLossLiability
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_IndemnificationGuaranteeMember		 $ 0ntrs_CreditLossLiability
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_IndemnificationGuaranteeMember