Share-Based Compensation Plans - Stock Options Exercised (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Stock Options Exercised
Intrinsic Value as of Exercise Date	$ 2.9	$ 1.6	$ 3.6
Cash Received	9.4	2.3	3.9
Tax Deduction Benefits Realized	$ 2.9	$ 1.6	$ 3.5

Employee Service Share Based Compensation Tax Benefit Realized From Exercise Of Stock Option

No definition available.

Name:	ntrs_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOption
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Share Based Compensation Arrangement by Share Based Payment Award, Options, Exercises in Period [Abstract]

+ References

No definition available.

+ Details

Name:	ntrs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodAbstract
Namespace Prefix:	ntrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash inflow from exercise of option under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 14
-Subparagraph (a)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-14

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 2A
-Subparagraph (a)
-SubTopic 10
-Topic 718
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2A

+ Details

Name:	us-gaap_ProceedsFromStockOptionsExercised
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration