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Changes in Level 3 Assets (Parenthetical) (Detail) (Auction Rate Securities, USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Realized gains (losses) on securities $ 4.4us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet  
Redemptions by Issuers
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Realized gains (losses) on securities 2.7us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ntrs_RedemptionsByIssuersMember
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AuctionRateSecuritiesMember		 0.1us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ntrs_RedemptionsByIssuersMember
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AuctionRateSecuritiesMember
Sales of Securities
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Realized gains (losses) on securities $ 1.7us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= ntrs_SalesOfSecuritiesMember
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AuctionRateSecuritiesMember