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Reclassification Adjustments out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Available for Sale Investment Securities | Realized (Gains) Losses on Securities Available for Sale | Investment Security Gains (Losses), net  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Gross Reclassification Adjustment $ (0.1)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ ntrs_ComponentsOfAccumulatedOtherComprehensiveIncomeAxis
= ntrs_RealizedGainLossOnSaleOfAvailableForSaleSecuritiesMember
/ us-gaap_IncomeStatementLocationAxis
= ntrs_AccumulatedNetRealizedInvestmentGainsLossesMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AvailableforsaleSecuritiesMember
CASH FLOW HEDGES | Foreign Exchange Contracts | Other Operating Income/Expense  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Gross Reclassification Adjustment 1.0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ ntrs_ComponentsOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= ntrs_OtherOperatingIncomeMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_CashFlowHedgingMember
Pension and Other Postretirement Benefit Adjustments  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Gross Reclassification Adjustment 9.5us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= ntrs_PensionAndOtherPostretirementAdjustmentsMember
Pension and Other Postretirement Benefit Adjustments | Employee Benefits  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Amortization of Net Actuarial (Gains) Losses 9.6us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax
/ us-gaap_IncomeStatementLocationAxis
= ntrs_EmployeeBenefitsMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= ntrs_PensionAndOtherPostretirementAdjustmentsMember
Amortization of Prior Service Cost $ (0.1)us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
/ us-gaap_IncomeStatementLocationAxis
= ntrs_EmployeeBenefitsMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= ntrs_PensionAndOtherPostretirementAdjustmentsMember