Derivative Financial Instruments (Fair Value of Securities Collateral for Derivative Transactions) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Not permitted by contract or custom to sell or repledge, pledged to others	$ 37.8	$ 39.9

This amount represents additional cash collateral received from derivative counterparties not offset against derivative assets on the consolidated balance sheet as amounts exceed the net derivative positions with the counterparties.

No definition available.

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