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Credit Related Impairment Losses Recognized in Earnings on Other Than Temporarily Impaired Securities (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Changes in OTTI Losses   $ (4.6)ntrs_OtherThanTemporaryImpairmentLosses [1]
Noncredit-related Losses Recorded in / (Reclassified from) OCI   0.7us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent [2]
Net Impairment Losses Recognized in Earnings $ 0ntrs_OtherThanTemporaryImpairmentLossesRecognizedInEarnings $ (3.9)ntrs_OtherThanTemporaryImpairmentLossesRecognizedInEarnings
[1] For initial OTTI in the applicable period, the balance includes the excess of the amortized cost over the fair value of the impaired securities. For subsequent impairments of the same security, the balance includes any additional changes in fair value of the security subsequent to its most recently recorded OTTI.
[2] For initial OTTI in the applicable period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in other comprehensive income (OCI). For subsequent impairments of the same security, the balance includes additional changes in OCI for that security subsequent to its most recently recorded OTTI.