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Summary of Book Values of Outstanding Subordinated Debentures (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Debt Outstanding [Line Items]    
Floating Rate Capital Debt $ 277.2ntrs_TrustPreferredSecurities $ 277.1ntrs_TrustPreferredSecurities
NTC Capital I Subordinated Debentures Due January 15th 2027    
Debt Outstanding [Line Items]    
Floating Rate Capital Debt 154.0ntrs_TrustPreferredSecurities
/ us-gaap_DebtInstrumentAxis
= ntrs_NtcCapitalISubordinatedDebenturesDueJanuaryFifteenTwentyTwentySevenMember		 154.0ntrs_TrustPreferredSecurities
/ us-gaap_DebtInstrumentAxis
= ntrs_NtcCapitalISubordinatedDebenturesDueJanuaryFifteenTwentyTwentySevenMember
NTC Capital II Subordinated Debentures Due April 15th 2027    
Debt Outstanding [Line Items]    
Floating Rate Capital Debt $ 123.2ntrs_TrustPreferredSecurities
/ us-gaap_DebtInstrumentAxis
= ntrs_NtcCapitalIISubordinatedDebenturesDueAprilFifteenTwentyTwentySevenMember		 $ 123.1ntrs_TrustPreferredSecurities
/ us-gaap_DebtInstrumentAxis
= ntrs_NtcCapitalIISubordinatedDebenturesDueAprilFifteenTwentyTwentySevenMember