Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loans and Leases
Specifically Evaluated for Impairment	$ 157.3	$ 242.2
Evaluated for Inherent Impairment	33,023.6	31,398.0
Total Loans and Leases	33,180.9	31,640.2
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	3.1	6.6
Evaluated for Inherent Impairment	190.7	260.4
Allowance Assigned to Loans and Leases	193.8	267.0	$ 278.1
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	39.5	28.9	29.8
Total Allowance for Credit Losses	233.3	295.9	307.9	$ 327.6
Commercial
Loans and Leases
Specifically Evaluated for Impairment	56.3	78.9
Evaluated for Inherent Impairment	15,100.2	14,274.7
Total Loans and Leases	15,156.5	14,353.6
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	3.0	5.8
Evaluated for Inherent Impairment	83.3	138.0
Allowance Assigned to Loans and Leases	86.3	143.8	140.9
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	28.5	25.9	27.1
Total Allowance for Credit Losses	114.8	169.7	168.0	194.2
Personal
Loans and Leases
Specifically Evaluated for Impairment	101.0	163.3
Evaluated for Inherent Impairment	17,923.4	17,123.3
Total Loans and Leases	18,024.4	17,286.6
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	0.1	0.8
Evaluated for Inherent Impairment	107.4	122.4
Allowance Assigned to Loans and Leases	107.5	123.2	137.2
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	11.0	3.0	2.7
Total Allowance for Credit Losses	$ 118.5	$ 126.2	$ 139.9	$ 133.4

Allowance for Undrawn Exposure [Abstract]

No definition available.

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

- Definition

This represents total allowance for credit losses established to cover probable credit losses related to loans and leases as well as off-balance sheet instruments including unfunded commitments and standby letters of credit. This differs from the US XBRL taxonomy tag in that the taxonomy tag does not include the allowance for off-balance sheet instruments in its scope.

+ References

No definition available.

+ Details

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

- Definition

Reflects the allowance for credit losses assigned to unfunded commitments and standby letters of credit as of the balance sheet date.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 6
-Section L
-Subsection 1

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524

+ Details

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

- References

No definition available.

+ Details

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

- Details

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

- Details

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