Earnings Contribution of Northern Trust's Reporting Segments (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 29.4	$ 32.5	$ 40.8

Represents Fully Taxable-Equivalent Adjustment (FTE). Net interest income stated on an FTE basis is a non-GAAP financial measure that facilitates the analysis of asset yields and provides a clearer indication of net interest margins for comparative purposes.

No definition available.

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