Business Units and Related Information - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Visa indemnified litigation benefit (charges) recorded	$ 0	$ 23.1	$ 33.0

Visa Indemnified Litigation Benefit (Charges) Recorded

No definition available.

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