Future Minimum Lease Payments to be Received (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Schedule Of Capital Leases Future Minimum Payments Receivable [Line Items]
2013	$ 48.6
2014	45.7
2015	39.5
2016	29.1
2017	$ 26.7

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

Amount of minimum lease payments to be received by the lessor in the next fiscal year following the latest fiscal year for capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6952072&loc=d3e45377-112738

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 23

-Subparagraph a(ii)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

Amount of minimum lease payments to be received by the lessor in the fifth fiscal year following the latest fiscal year for capital leases.

+ References

+ Details

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

- Definition

Amount of minimum lease payments to be received by the lessor in the fourth fiscal year following the latest fiscal year for capital leases.

+ References

+ Details

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

- Definition

Amount of minimum lease payments to be received by the lessor in the third fiscal year following the latest fiscal year for capital leases.

+ References

+ Details

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

- Definition

Amount of minimum lease payments to be received by the lessor in the second fiscal year following the latest fiscal year for capital leases.

+ References

+ Details

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