Earnings Contribution of Northern Trust's Business Units (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 7.9	$ 10.2	$ 15.5	$ 20.1

Segment Reporting Information, Fully Taxable-Equivalent Adjustment

No definition available.

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