Derivative Financial Instruments - Fair Value of Securities Collateral for Derivative Transactions (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Not permitted by contract or custom to sell or repledge	$ 39,900,000	$ 70,700,000
Permitted by contract or custom to sell or repledge	$ 4,600,000	$ 0

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