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Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 4,170.8us-gaap_HeldToMaturitySecurities $ 2,325.8us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains 26.4us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 12.3us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Gross Unrealized Losses 21.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 16.7us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Fair Value 4,176.1us-gaap_HeldToMaturitySecuritiesFairValue 2,321.4us-gaap_HeldToMaturitySecuritiesFairValue
Obligations of States and Political Subdivisions    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 121.9us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
225.2us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Gains 7.4us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
10.3us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value 129.3us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
235.5us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
U.S. Government Sponsored Agency    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 18.4us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
35.9us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Gross Unrealized Gains 1.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
1.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fair Value 19.5us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
37.0us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Non-U.S. Government    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,281.6us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
722.0us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Gross Unrealized Gains 6.6us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
0.8us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Gross Unrealized Losses 0.4us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
1.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fair Value 1,287.8us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
721.7us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Certificates of Deposit    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 924.3us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
698.1us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
Gross Unrealized Gains 0.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
 
Gross Unrealized Losses 0.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
0.2us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
Fair Value 924.3us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
697.9us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_CertificatesOfDepositMember
Supranational and Non-U.S. Agency Bonds    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,745.8us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
584.7us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
Gross Unrealized Gains 10.9us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
 
Gross Unrealized Losses 0.5us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
3.2us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
Fair Value 1,756.2us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
581.5us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_SupranationalAndNonUsAgencyBondsMember
Others    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 78.8us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
59.9us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
Gross Unrealized Gains 0.3us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
0.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
Gross Unrealized Losses 20.1us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
12.2us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
Fair Value $ 59.0us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember
$ 47.8us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= ntrs_OtherSecuritiesMember