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Commitments and Letters of Credit (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Commitments to Extend Credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Commitments and Letters of Credit $ 35,127.6us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_CommitmentsToExtendCreditMember		 [1] $ 32,174.8us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_CommitmentsToExtendCreditMember		 [1]
Standby Letters of Credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Commitments and Letters of Credit 4,468.1us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_StandbyLettersOfCreditMember		 [2] 4,451.1us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= us-gaap_StandbyLettersOfCreditMember		 [2]
Commercial Letters of Credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Commitments and Letters of Credit $ 20.8us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= ntrs_CommercialLettersOfCreditMember		 $ 24.8us-gaap_FairValueDisclosureOffbalanceSheetRisksAmountLiability
/ us-gaap_LineOfCreditFacilityAxis
= ntrs_CommercialLettersOfCreditMember
[1] These amounts exclude $481.4 million and $418.5 million of commitments participated to others at December 31, 2014 and 2013, respectively.
[2] These amounts include $221.4 million and $208.9 million of standby letters of credit secured by cash deposits or participated to others as of December 31, 2014 and 2013, respectively. The weighted average maturity of standby letters of credit was 27 months at December 31, 2014 and 25 months at December 31, 2013.