Line of credit draw down period for equity credit line products.

No definition available.

Name:	ntrs_LineOfCreditDrawDownPeriod
Namespace Prefix:	ntrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans and Leases Receivable, Commitment to Lend

+ References

No definition available.

+ Details

Name:	ntrs_LoansAndLeasesReceivableCommitmentToLend
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans and Leases Receivable, Non Performing Loans, Interest That Would Have Been Recognized

+ References

No definition available.

+ Details

Name:	ntrs_LoansAndLeasesReceivableNonPerformingLoansInterestThatWouldHaveBeenRecognized
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Describes the policy as to when a loan ceases to accrue interest or other revenue because the borrower is in financial difficulty. Also includes the policy for determining past-due or delinquency status (i.e. whether past-due status is based on how recently payments have been received or on contractual terms).

+ References

No definition available.

+ Details

Name:	ntrs_LoansAndLeasesReceivableNonaccrualLoanAndLeasePolicy
Namespace Prefix:	ntrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents short duration advances, primarily related to overdrafts associated with the timing of custody client's investments included in other U.S. and non-U.S. loans.

+ References

No definition available.

+ Details

Name:	ntrs_LoansReceivableOverdraftAdvances
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans and Leases Receivable to Performing Status

+ References

No definition available.

+ Details

Name:	ntrs_LoansandLeasesReceivabletoPerformingStatus
Namespace Prefix:	ntrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lower Limit of Generally Required Loan to Collateral Value for Residential Real Estate Loans

+ References

No definition available.

+ Details

Name:	ntrs_LowerLimitOfGenerallyRequiredLoanToCollateralValueForResidentialRealEstateLoans
Namespace Prefix:	ntrs_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Payment Periods

+ References

No definition available.

+ Details

Name:	ntrs_NumberofPaymentPeriods
Namespace Prefix:	ntrs_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of overdrawn money in accounts that may bear interest and that the depositor is entitled to withdraw at any time without prior notice, which have been reclassified to loans.

+ References

No definition available.

+ Details

Name:	ntrs_OverdrawnDemandDepositAccounts
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of Total Equity Credit Lines

+ References

No definition available.

+ Details

Name:	ntrs_PercentageofTotalEquityCreditLines
Namespace Prefix:	ntrs_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individual homogeneous loans under this dollar threshold are collectively evaluated for impairment and excluded from impaired loan disclosures.

+ References

No definition available.

+ Details

Name:	ntrs_SmallerHomogeneousLoanThreshold
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Troubled Debt Restructuring Loans and Leases Receivable to Performing Status, Minimum Period of Time

+ References

No definition available.

+ Details

Name:	ntrs_TroubledDebtRestructuringLoansandLeasesReceivabletoPerformingStatusMinimumPeriodofTime
Namespace Prefix:	ntrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Upper Limit of Generally Required Loan to Collateral Value for Residential Real Estate loans

+ References

No definition available.

+ Details

Name:	ntrs_UpperLimitOfGenerallyRequiredLoanToCollateralValueForResidentialRealEstateLoans
Namespace Prefix:	ntrs_
Data Type:	num:percentItemType
Balance Type:	na
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_AccountsNotesAndLoansReceivableLineItems
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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-Section S99
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Amount before allowance of consumer real estate loans. Includes, but not limited to, construction financing, residential mortgage loans, and home equity lines of credit. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a)(2),(3),(b))
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-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
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-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

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

- Definition

Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

+ Details

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

- Definition

Amount, after allowance for bad debt, of other loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

+ Details

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

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_NonperformingFinancingReceivableMember
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Period Type:

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_PerformingFinancingReceivableMember
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- Details

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- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
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Period Type:

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Loans and Leases (Narrative) (Details)	3 Months Ended		12 Months Ended
Loans and Leases (Narrative) (Details)	Mar. 31, 2019 USD ($) payment_period	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Loan	Dec. 31, 2017 USD ($) Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%
Collateral required on Residential real estate loan, upper limit	80.00%
Draw period for credit line products	10 years
Other U.S. loans and non-U.S. loans, short duration advances	$ 1,200,000,000		$ 2,200,000,000
Demand deposits	88,400,000		152,500,000
Loans classified as held for sale	0
Leases classified as held for sale			0
Interest income that would have been recorded for nonperforming loans	1,900,000	$ 2,200,000
Undrawn loan commitments and standby letters of credit	9,000,000		12,600,000
Total loans and leases	$ 30,620,600,000		32,490,000,000
Performing status, minimum period of time	6 months
Recorded investment		$ 400,000		$ 300,000
Consumer loans with carrying value collateralized by residential real estate property	$ 11,800,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	70,900,000		64,600,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 33,200,000		$ 35,200,000
Re-Defaulted \| Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans and leases \| Loan			0	2
Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection	90 days
Lower Limit \| Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan	6 months
Upper Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of payment periods \| payment_period	6
Limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures (less than)	$ 1,000,000
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 526,700,000		$ 655,500,000
Percentage of total equity credit lines	96.00%		95.00%