Loans and Leases (Troubled Debt Restructurings) (Details) $ in Millions	12 Months Ended
Dec. 31, 2021 USD ($) contract	Dec. 31, 2020 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	19	25
RECORDED INVESTMENT	$ 56.9	$ 40.5
UNPAID PRINCIPAL BALANCE	63.5	$ 41.2
Residential Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
RECORDED INVESTMENT	5.6
UNPAID PRINCIPAL BALANCE	$ 5.6
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	9	3
RECORDED INVESTMENT	$ 55.6	$ 24.3
UNPAID PRINCIPAL BALANCE	$ 62.0	$ 24.5
Commercial \| Commercial and Institutional
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	6	3
RECORDED INVESTMENT	$ 19.6	$ 24.3
UNPAID PRINCIPAL BALANCE	$ 19.6	$ 24.5
Commercial \| Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	3
RECORDED INVESTMENT	$ 36.0
UNPAID PRINCIPAL BALANCE	$ 42.4
Personal
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	10	22
RECORDED INVESTMENT	$ 1.3	$ 16.2
UNPAID PRINCIPAL BALANCE	$ 1.5	$ 16.7
Personal \| Residential Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
NUMBER OF COVID-19 RELATED MODIFICATIONS \| contract	10	22
RECORDED INVESTMENT	$ 1.3	$ 16.2
UNPAID PRINCIPAL BALANCE	$ 1.5	$ 16.7

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 http://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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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: