Securities - Remaining Maturity of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available for Sale - Fair Value
U.S. Government	$ 8,882.1
Obligations of States and Political Subdivisions	18,311.7
Government Sponsored Agency	9,005.2
Non-U.S. Government	2,677.3
Total	38,876.3	$ 36,888.8
Available for Sale - Amortized Cost
U.S. Government	8,870.5
Obligations of States and Political Subdivisions	18,195.7
Government Sponsored Agency	8,977.4
Non-U.S. Government	2,678.6
AMORTIZED COST	38,722.2	37,041.8
U.S. Government
Available for Sale - Fair Value
U.S. Government	1,898.4
Obligations of States and Political Subdivisions	2,098.3
Government Sponsored Agency	552.4
Non-U.S. Government	0.0
Total	4,549.1	5,185.3
Available for Sale - Amortized Cost
U.S. Government	1,899.0
Obligations of States and Political Subdivisions	2,075.9
Government Sponsored Agency	552.6
Non-U.S. Government	0.0
AMORTIZED COST	4,527.5	5,203.1
Obligations of States and Political Subdivisions
Available for Sale - Fair Value
U.S. Government	80.1
Obligations of States and Political Subdivisions	85.4
Government Sponsored Agency	1,449.8
Non-U.S. Government	0.0
Total	1,615.3	655.9
Available for Sale - Amortized Cost
U.S. Government	80.0
Obligations of States and Political Subdivisions	83.0
Government Sponsored Agency	1,441.0
Non-U.S. Government	0.0
AMORTIZED COST	1,604.0	657.6
Government Sponsored Agency
Available for Sale - Fair Value
U.S. Government	5,005.0
Obligations of States and Political Subdivisions	9,728.8
Government Sponsored Agency	5,869.4
Non-U.S. Government	2,668.0
Total	23,271.2	22,424.6
Available for Sale - Amortized Cost
U.S. Government	4,994.1
Obligations of States and Political Subdivisions	9,714.1
Government Sponsored Agency	5,870.0
Non-U.S. Government	2,669.3
AMORTIZED COST	23,247.5	22,522.7
Non-U.S. Government
Available for Sale - Fair Value
U.S. Government	0.0
Obligations of States and Political Subdivisions	3.3
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
Total	3.3	142.2
Available for Sale - Amortized Cost
U.S. Government	0.0
Obligations of States and Political Subdivisions	3.3
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
AMORTIZED COST	3.3	143.3
Corporate Debt
Available for Sale - Fair Value
U.S. Government	341.7
Obligations of States and Political Subdivisions	2,005.5
Government Sponsored Agency	55.5
Non-U.S. Government	0.0
Total	2,402.7	2,294.7
Available for Sale - Amortized Cost
U.S. Government	341.8
Obligations of States and Political Subdivisions	1,981.5
Government Sponsored Agency	55.6
Non-U.S. Government	0.0
AMORTIZED COST	2,378.9	2,312.6
Covered Bonds
Available for Sale - Fair Value
U.S. Government	281.6
Obligations of States and Political Subdivisions	488.3
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
Total	769.9	829.3
Available for Sale - Amortized Cost
U.S. Government	281.0
Obligations of States and Political Subdivisions	485.3
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
AMORTIZED COST	766.3	832.7
Certificates of Deposit
Available for Sale - Fair Value
U.S. Government	334.6
Obligations of States and Political Subdivisions	1,743.1
Government Sponsored Agency	49.9
Non-U.S. Government	0.0
Total	2,127.6
Available for Sale - Amortized Cost
U.S. Government	333.9
Obligations of States and Political Subdivisions	1,707.4
Government Sponsored Agency	50.0
Non-U.S. Government	0.0
AMORTIZED COST	2,091.3
Other Asset-Backed
Available for Sale - Fair Value
U.S. Government	885.3
Obligations of States and Political Subdivisions	1,984.8
Government Sponsored Agency	451.1
Non-U.S. Government	9.3
Total	3,330.5	2,657.7
Available for Sale - Amortized Cost
U.S. Government	885.1
Obligations of States and Political Subdivisions	1,977.3
Government Sponsored Agency	452.8
Non-U.S. Government	9.3
AMORTIZED COST	3,324.5	2,678.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Fair Value
Total	2,127.6	2,096.2
Available for Sale - Amortized Cost
AMORTIZED COST	2,091.3	2,087.8
Commercial Mortgage-Backed
Available for Sale - Fair Value
U.S. Government	46.4
Obligations of States and Political Subdivisions	174.2
Government Sponsored Agency	577.1
Non-U.S. Government	0.0
Total	797.7	587.2
Available for Sale - Amortized Cost
U.S. Government	46.6
Obligations of States and Political Subdivisions	167.9
Government Sponsored Agency	555.4
Non-U.S. Government	0.0
AMORTIZED COST	769.9	587.4
Other
Available for Sale - Fair Value
U.S. Government	9.0
Obligations of States and Political Subdivisions	0.0
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
Total	9.0	15.7
Available for Sale - Amortized Cost
U.S. Government	9.0
Obligations of States and Political Subdivisions	0.0
Government Sponsored Agency	0.0
Non-U.S. Government	0.0
AMORTIZED COST	$ 9.0	$ 15.7

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth fiscal year through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ 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 (d)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

+ Details

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

- Definition

Amount 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/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- 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=us-gaap_CertificatesOfDepositMember
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=ntrs_SubSovereignSupranationalAndNonUsAgencyBondsMember
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:

- Details

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