Loans (Loan Modifications Made To Borrowers Experiencing Financial Difficulty) (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2023
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 21,500,000	$ 24,600,000
% OF TOTAL SEGMENT	0.05%	0.06%
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 16,700,000	$ 16,700,000
% OF TOTAL SEGMENT	0.08%	0.08%
Commercial \| Commercial and Institutional \| Extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 16,700,000	$ 16,700,000
% OF TOTAL SEGMENT	0.14%	0.14%
Financing receivable, modified, weighted average term increase from modification	12 months	12 months
Commercial \| Commercial Real Estate \| Combination of principal and/or interest deferral and extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 0	$ 0
% OF TOTAL SEGMENT	0.00%	0.00%
Commercial \| Commercial Real Estate \| Principal and/or interest deferral
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, excluding accrued interest, modified, deferred interest and principal		$ 32,500,000
Financing receivable, modified, weighted average term increase from modification		6 months
Commercial \| Commercial Real Estate \| Extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, weighted average term increase from modification		6 months
Personal
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 4,800,000	$ 7,900,000
% OF TOTAL SEGMENT	0.02%	0.04%
Financing receivable, excluding accrued interest, modified in period, loans with no outstanding balance, amount		$ 0
Personal \| Commercial Real Estate \| Combination of principal and/or interest deferral and extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, excluding accrued interest, modified, deferred interest and principal		32,500,000
Personal \| Residential Real Estate \| Combination of principal and/or interest deferral and extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 4,700,000	$ 4,700,000
% OF TOTAL SEGMENT	0.08%	0.08%
Personal \| Residential Real Estate \| Principal and/or interest deferral
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 100,000	$ 3,200,000
% OF TOTAL SEGMENT	0.00%	0.05%
Financing receivable, modified, weighted average term increase from modification	9 months	9 months
Personal \| Residential Real Estate \| Extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, modified, weighted average term increase from modification	9 months	9 months
Personal \| Private Client \| Extension of term
Financing Receivable, Troubled Debt Restructuring [Line Items]
AMORTIZED COST BASIS	$ 0	$ 0
% OF TOTAL SEGMENT	0.00%	0.00%
Financing receivable, excluding accrued interest, modified, deferred interest and principal		$ 200,000
Financing receivable, modified, weighted average term increase from modification		60 months

Financing Receivable, Excluding Accrued Interest, Modified, Financially Distressed

No definition available.

Name:	ntrs_FinancingReceivableExcludingAccruedInterestModifiedFinanciallyDistressed
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Financing Receivable, Excluding Accrued Interest, Modified in Period, Loans With No Outstanding Balance, Amount

+ References

No definition available.

+ Details

Name:	ntrs_FinancingReceivableExcludingAccruedInterestModifiedInPeriodLoansWithNoOutstandingBalanceAmount
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

+ Details

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

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ntrs_CommercialAndInstitutionalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=ntrs_PaymentDeferralAndExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ntrs_PrivateClientMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: