Loans and Leases (Troubled Debt Restructurings) (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2022 USD ($) contract	Sep. 30, 2021 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	3	1	6	16
RECORDED INVESTMENT	$ 31.8	$ 0.2	$ 32.4	$ 48.9
UNPAID PRINCIPAL BALANCE	$ 32.7	$ 0.2	$ 33.3	$ 55.3
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	2	0	3	8
RECORDED INVESTMENT	$ 31.7	$ 0.0	$ 32.2	$ 47.9
UNPAID PRINCIPAL BALANCE	$ 32.6	$ 0.0	$ 33.1	$ 54.1
Commercial \| Commercial and Institutional
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	1	0	2	6
RECORDED INVESTMENT	$ 0.1	$ 0.0	$ 0.6	$ 19.6
UNPAID PRINCIPAL BALANCE	$ 0.1	$ 0.0	$ 0.6	$ 19.6
Commercial \| Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	1	0	1	2
RECORDED INVESTMENT	$ 31.6	$ 0.0	$ 31.6	$ 28.3
UNPAID PRINCIPAL BALANCE	$ 32.5	$ 0.0	$ 32.5	$ 34.5
Personal
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	1	1	3	8
RECORDED INVESTMENT	$ 0.1	$ 0.2	$ 0.2	$ 1.0
UNPAID PRINCIPAL BALANCE	$ 0.1	$ 0.2	$ 0.2	$ 1.2
Personal \| Residential Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF LOANS AND LEASES \| contract	1	1	3	8
RECORDED INVESTMENT	$ 0.1	$ 0.2	$ 0.2	$ 1.0
UNPAID PRINCIPAL BALANCE	$ 0.1	$ 0.2	$ 0.2	$ 1.2

Financing Receivable, Troubled Debt Restructuring, Postmodification, Unpaid Principal Balance

No definition available.

Name:	ntrs_FinancingReceivableTroubledDebtRestructuringPostmodificationUnpaidPrincipalBalance
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
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: