Earnings Contribution of Northern Trust's Reporting Segments (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 6.2	$ 8.7

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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Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration