Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Other liabilities	$ 2,893.4	$ 2,338.4

Other Liabilities Excluding Derivatives And Non Financial Instruments

No definition available.

Name:	ntrs_OtherLiabilitiesExcludingDerivativesAndNonFinancialInstruments
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant