Concentrations of Credit Risk - Narrative (Detail) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Credit risk to banks	$ 6.9	$ 6.8
Credit risk to banks - non-interest bearing balances held at banks	2.1	1.9
Residential Real Estate	$ 7.2	$ 8.1
Residential real estate loans as percentage of total U.S. loans	23.00%	25.00%
Lower limit of generally required loan to collateral value for residential real estate loans	65.00%
Upper limit of generally required loan to collateral value for residential real estate loans	80.00%
Legally binding undrawn commitments to extend credit	$ 1.0	$ 1.2
Florida
Concentration Risk [Line Items]
Residential Real Estate	1.9
Greater Chicago Area
Concentration Risk [Line Items]
Residential Real Estate	1.4
California
Concentration Risk [Line Items]
Residential Real Estate	$ 1.4

- Definition

This represents the amount of legally binding commitments to extend residential real estate credit as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	ntrs_CommitmentsToExtendCredit
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents on-balance sheet credit risk to banks consisting primarily of interest bearing deposits with banks, federal funds sold, and securities purchased under agreements to resell.

+ References

No definition available.

+ Details

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

- Definition

Credit Risk to Banks Non Interest Bearing Balances Held at Banks

+ References

No definition available.

+ Details

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

- Definition

Loans and Leases Receivable, Residential, Real Estate

+ References

No definition available.

+ Details

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

- 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

Residential Real Estate Loans Percentage Of Total US Loans

+ References

No definition available.

+ Details

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

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

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_FL
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=ntrs_GreaterChicagoAreaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: