Earnings Contribution of Northern Trust's Business Units (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 11.3	$ 9.8	$ 31.4	$ 30.8

Segment Reporting Information, Fully Taxable-Equivalent Adjustment

No definition available.

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