Restrictions on Subsidiary Dividends and Loans or Advances - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Restrictions on Subsidiary Dividends and Loans or Advances [Line Items]
Additional amount of dividends that could be declare without regulatory approval	$ 328.8
Regulatory limits on transfer of capital and surplus by a banking subsidiary to the Corporation and certain of its affiliates	10.00%
Regulatory limits on aggregate transfer of capital and surplus by banking subsidiaries to the Corporation	20.00%

The additional amount of dividends that may be paid or declared in the following calendar year in addition to the following years eligible net profits without prior approval of regulatory agencies.

No definition available.

Name:	ntrs_AdditionalAmountAvailableForDividendDistributionWithoutPriorApprovalFromRegulatoryAgency
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant