Earnings Contribution of Northern Trust's Business Units (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$ 9.9	$ 10.5

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