Number of Loans and Leases Modified in TDRs and Total Recorded Investments and Unpaid Principal Balances (Detail) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	10	0
Financing Receivable
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	136	138
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 54.9	$ 19.8
Financing Receivable, Modifications, Unpaid Principal Balance	$ 62.9	$ 24.0
Financing Receivable \| Commercial
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	7	10
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 26.4	$ 4.6
Financing Receivable, Modifications, Unpaid Principal Balance	$ 28.1	$ 5.6
Financing Receivable \| Commercial \| Commercial and Institutional
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	3	2
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 24.6	$ 0.7
Financing Receivable, Modifications, Unpaid Principal Balance	$ 26.1	$ 0.8
Financing Receivable \| Commercial \| Commercial Real Estate
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	4	8
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 1.8	$ 3.9
Financing Receivable, Modifications, Unpaid Principal Balance	$ 2.0	$ 4.8
Financing Receivable \| Personal
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	129	128
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 28.5	$ 15.2
Financing Receivable, Modifications, Unpaid Principal Balance	$ 34.8	$ 18.4
Financing Receivable \| Personal \| Residential Real Estate
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	128	124
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 27.9	$ 15.0
Financing Receivable, Modifications, Unpaid Principal Balance	$ 34.2	$ 17.9
Financing Receivable \| Personal \| Private Client
Financing Receivable, Modifications [Line Items]
Loans or leases modified in trouble debt restructuring \| Contract	1	4
Financing Receivable, Modifications, Recorded Investment, Modified During Period	$ 0.6	$ 0.2
Financing Receivable, Modifications, Unpaid Principal Balance	$ 0.6	$ 0.5

Financing Receivable, Modifications, Recorded Investment, Modified During Period

No definition available.

Name:	ntrs_FinancingReceivableModificationsRecordedInvestmentModifiedDuringPeriod
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Financing Receivable, Modifications, Unpaid Principal Balance

+ References

No definition available.

+ Details

Name:	ntrs_FinancingReceivableModificationsUnpaidPrincipalBalance
Namespace Prefix:	ntrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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_FinancingReceivableModificationsLineItems
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

+ Details

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

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_FinanceReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ntrs_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableSecondaryAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ntrs_CommercialAndInstitutionalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ntrs_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableSecondaryAxis=ntrs_PersonalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ntrs_PrivateClientMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: