Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Investment Holdings [Line Items]
AMORTIZED COST	$ 28,269.6	$ 37,948.5
GROSS UNREALIZED GAINS	9.0	310.6
GROSS UNREALIZED LOSSES	1,299.3	248.6
FAIR VALUE	26,979.3	38,010.5
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,641.3	2,406.2
GROSS UNREALIZED GAINS	2.6	29.3
GROSS UNREALIZED LOSSES	99.9	9.4
FAIR VALUE	2,544.0	2,426.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	963.4	3,841.0
GROSS UNREALIZED GAINS	0.0	73.7
GROSS UNREALIZED LOSSES	125.5	38.6
FAIR VALUE	837.9	3,876.1
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	12,192.9	18,092.1
GROSS UNREALIZED GAINS	5.7	93.7
GROSS UNREALIZED LOSSES	299.9	110.2
FAIR VALUE	11,898.7	18,075.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	305.2	383.4
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	28.6	9.5
FAIR VALUE	276.6	374.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	1,943.6	2,319.8
GROSS UNREALIZED GAINS	0.2	31.6
GROSS UNREALIZED LOSSES	103.3	9.7
FAIR VALUE	1,840.5	2,341.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	427.4	502.6
GROSS UNREALIZED GAINS	0.1	3.9
GROSS UNREALIZED LOSSES	16.0	0.9
FAIR VALUE	411.5	505.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,662.1	3,052.4
GROSS UNREALIZED GAINS	0.3	28.1
GROSS UNREALIZED LOSSES	174.1	35.4
FAIR VALUE	2,488.3	3,045.1
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	5,636.7	5,962.0
GROSS UNREALIZED GAINS	0.0	11.3
GROSS UNREALIZED LOSSES	342.7	31.7
FAIR VALUE	5,294.0	5,941.6
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	1,497.0	1,389.0
GROSS UNREALIZED GAINS	0.1	38.9
GROSS UNREALIZED LOSSES	109.3	3.2
FAIR VALUE	$ 1,387.8	$ 1,424.7

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ntrs_CoveredBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ntrs_SubSovereignSupranationalAndNonUsAgencyBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ntrs_OtherAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: