Earnings Contribution of Northern Trust's Business Units (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 40.2	$ 39.1	$ 40.2

Segment Reporting Information, Fully Taxable-Equivalent Adjustment

No definition available.

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