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Components of Other Operating Income (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Components of Other Operating Income [Line Items]      
Loan Service Fees $ 62.7us-gaap_FeesAndCommissionsMortgageBanking $ 61.9us-gaap_FeesAndCommissionsMortgageBanking $ 64.5us-gaap_FeesAndCommissionsMortgageBanking
Banking Service Fees 49.6ntrs_BankingServiceFees 50.9ntrs_BankingServiceFees 55.0ntrs_BankingServiceFees
Other Income 41.2us-gaap_FeesAndCommissionsOther 53.7us-gaap_FeesAndCommissionsOther 35.4us-gaap_FeesAndCommissionsOther
Total Other Operating Income $ 153.5us-gaap_NoninterestIncomeOtherOperatingIncome $ 166.5us-gaap_NoninterestIncomeOtherOperatingIncome $ 154.9us-gaap_NoninterestIncomeOtherOperatingIncome