Floating Rate Capital Debt - Subordinated Debentures (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Outstanding [Line Items]
Floating Rate Capital Debt	$ 277.5	$ 277.4
NTC Capital I Subordinated Debentures due January 15, 2027
Debt Outstanding [Line Items]
Floating Rate Capital Debt	154.2	154.1
NTC Capital II Subordinated Debentures due April 15, 2027
Debt Outstanding [Line Items]
Floating Rate Capital Debt	$ 123.3	$ 123.3

Amount of floating rate capital debt held by the company that have an unconditional obligation requiring the issuer to redeem the securities by transferring the assets at a specified or determinable date (or dates) or upon an event that is certain to occur. The debt was issued by a business trust to outside investors where the trust uses the proceeds of the issuance of those securities to purchase, from the financial institution that established the trust, an equivalent amount of junior subordinated debentures or other loans having stated maturities. The floating rate capital debt must be redeemed upon maturity of the debentures or other loans.

No definition available.

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