Accumulated Other Comprehensive Income (Loss) (Details of Changes In Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
BEFORE TAX
Net Change	$ 338.3	$ 149.2
TAX EFFECT
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	98.6	40.8
AFTER TAX
Net Change	239.7	108.4
Unrealized Gains (Losses) on Debt Securities Available for Sale
BEFORE TAX
Unrealized Gains (Losses) on Debt Securities Available for Sale	266.8	115.3
Reclassification Adjustment for (Gains) Losses Included in Net Income	0.0	0.2
Net Change	266.8	115.5
TAX EFFECT
Unrealized Gains (Losses) on Debt Securities Available for Sale	(67.1)	(29.9)
Reclassification Adjustment for (Gains) Losses Included in Net Income	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	67.1	29.9
AFTER TAX
OCI, before Reclassifications, Net of Tax, Attributable to Parent	199.7	85.4
Reclassification Adjustment for (Gains) Losses Included in Net Income	0.0	0.2
Net Change	199.7	85.6
Unrealized Gains (Losses) on Cash Flow Hedges
BEFORE TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	(4.0)	(13.6)
Net Change	9.7	(5.8)
TAX EFFECT
Reclassification Adjustment for (Gains) Losses Included in Net Income	1.0	3.4
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	2.4	(1.5)
AFTER TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	(3.0)	(10.2)
Net Change	7.3	(4.3)
Foreign Currency Adjustments
BEFORE TAX
Net Change	52.3	22.8
TAX EFFECT
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	26.6	8.0
AFTER TAX
Net Change	25.7	14.8
Foreign Currency Translation Adjustments
BEFORE TAX
Net Change	(89.8)	(6.5)
TAX EFFECT
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(9.6)	(1.6)
AFTER TAX
Net Change	(80.2)	(4.9)
Long-Term Intra-Entity Foreign Currency Transaction Gains (Losses)
BEFORE TAX
Net Change	(0.6)	0.7
TAX EFFECT
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.1)	0.2
AFTER TAX
Net Change	(0.5)	0.5
Net Investment Hedge Gains (Losses)
BEFORE TAX
Net Change	142.7	28.6
TAX EFFECT
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	36.3	9.4
AFTER TAX
Net Change	106.4	19.2
Pension and Other Postretirement Benefit Adjustments
BEFORE TAX
Unrealized Gains (Losses) on Debt Securities Available for Sale	(1.3)	11.3
Net Change	9.5	16.7
TAX EFFECT
Unrealized Gains (Losses) on Debt Securities Available for Sale	0.1	(3.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	2.5	4.4
AFTER TAX
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(1.2)	8.2
Net Change	7.0	12.3
Amortization of Net Actuarial Loss
BEFORE TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	10.9	5.5
TAX EFFECT
Reclassification Adjustment for (Gains) Losses Included in Net Income	(2.6)	(1.3)
AFTER TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	8.3	4.2
Amortization of Prior Service Cost
BEFORE TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	(0.1)	(0.1)
TAX EFFECT
Reclassification Adjustment for (Gains) Losses Included in Net Income	0.0	0.0
AFTER TAX
Reclassification Adjustment for (Gains) Losses Included in Net Income	(0.1)	(0.1)
Foreign Exchange Contracts \| Unrealized Gains (Losses) on Cash Flow Hedges
BEFORE TAX
Unrealized Gains (Losses) on Debt Securities Available for Sale	13.3	7.2
TAX EFFECT
Unrealized Gains (Losses) on Debt Securities Available for Sale	(3.3)	(1.7)
AFTER TAX
OCI, before Reclassifications, Net of Tax, Attributable to Parent	10.0	5.5
Interest Rate Contracts \| Unrealized Gains (Losses) on Cash Flow Hedges
BEFORE TAX
Unrealized Gains (Losses) on Debt Securities Available for Sale	0.4	0.6
TAX EFFECT
Unrealized Gains (Losses) on Debt Securities Available for Sale	(0.1)	(0.2)
AFTER TAX
OCI, before Reclassifications, Net of Tax, Attributable to Parent	$ 0.3	$ 0.4

AOCI Attributable To Parent, After Tax [Abstract]

No definition available.

Name:	ntrs_AOCIAttributableToParentAfterTaxAbstract
Namespace Prefix:	ntrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

AOCI Attributable To Parent, Before Tax [Abstract]

+ References

No definition available.

+ Details

Name:	ntrs_AOCIAttributableToParentBeforeTaxAbstract
Namespace Prefix:	ntrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

AOCI Attributable To Parent, Tax [Abstract]

+ References

No definition available.

+ Details

Name:	ntrs_AOCIAttributableToParentTaxAbstract
Namespace Prefix:	ntrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

+ References

+ Details

Name:	us-gaap_OciBeforeReclassificationsBeforeTaxAttributableToParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580

+ Details

Name:	us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e39076-109324

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

+ Details

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

- Definition

Amount before tax of other comprehensive income (loss) attributable to parent entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569643-111683

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after tax of other comprehensive income (loss) attributable to parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669625-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569643-111683

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669619-108580

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569643-111683

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount before tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

+ References

+ Details

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

- Definition

Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

+ Details

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

- Definition

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL34724391-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e39076-109324

+ Details

Name:	us-gaap_ReclassificationFromAociCurrentPeriodTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedTranslationAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=ntrs_AccumulatedForeignCurrencyAdjustmentAttributabletoParentOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=ntrs_LongtermIntraentityForeignCurrencyTransactionGainsLossesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=ntrs_NetInvestmentHedgeGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: