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Pledged Assets - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged $ 33,200,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue   $ 32,300,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
Fair value of accepted collateral 1,000,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue   1,000,000,000us-gaap_SecuritiesHeldAsCollateralAtFairValue
Amount of securities repledged or sold collateral 0us-gaap_CollateralSecuritiesRepledgedDeliveredOrUsed   0us-gaap_CollateralSecuritiesRepledgedDeliveredOrUsed
Average deposits maintained to meet Federal Reserve Bank reserve requirements 1,500,000,000ntrs_AverageInterestBearingDepositsInFederalReserveBankForReserveRequirements 1,200,000,000ntrs_AverageInterestBearingDepositsInFederalReserveBankForReserveRequirements  
US Government Corporations and Agencies Securities      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged 23,900,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
  23,100,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Obligations of States and Political Subdivisions      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged 113,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
  122,900,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Loans      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged 9,200,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_LoansMember
  9,100,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_LoansMember
Collateral Requirements      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged 5,600,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= ntrs_CollateralRequirementsMember
  5,900,000,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= ntrs_CollateralRequirementsMember
Available for Sale Investment Securities      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Amount of securities and loans pledged $ 611,400,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_AvailableforsaleSecuritiesMember
  $ 884,800,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ ntrs_PledgedAndRestrictedAssetsAxis
= us-gaap_AvailableforsaleSecuritiesMember