Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014 Segment
Significant Accounting Policies [Line Items]
Number of reporting segments	3us-gaap_NumberOfReportableSegments
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	$ 250,000ntrs_SmallerHomogeneousLoanThreshold
Lower Limit
Significant Accounting Policies [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
Loans and leases receivable nonperforming to performing status	6 months
Restructured loans and leases receivable nonperforming to performing status	6 months
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan	6 months
Troubled debt restructuring loans and leases receivable to Performing status	6 months
Lower Limit \| Equipment
Significant Accounting Policies [Line Items]
Charge for depreciation, range of life	3 years
Lower Limit \| Leasehold Improvements
Significant Accounting Policies [Line Items]
Charge for depreciation, range of life	The shorter of the lease term or 15 years.
Lower Limit \| Computer Software
Significant Accounting Policies [Line Items]
Software, useful life	3 years
Upper Limit
Significant Accounting Policies [Line Items]
Number of days past due for disclosure purposes to be categorized as a current	29 days
Upper Limit \| Buildings
Significant Accounting Policies [Line Items]
Charge for depreciation, range of life	30 years
Upper Limit \| Equipment
Significant Accounting Policies [Line Items]
Charge for depreciation, range of life	10 years
Upper Limit \| Leasehold Improvements
Significant Accounting Policies [Line Items]
Charge for depreciation, range of life	15 years
Upper Limit \| Computer Software
Significant Accounting Policies [Line Items]
Software, useful life	10 years
Wealth Management
Significant Accounting Policies [Line Items]
Services provided by business units, states	18
Minimum assets requirement to be managed	$ 200,000,000

Current Loans and Leases Policy Period

No definition available.

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- 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

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- Definition

Policy for minimum period of time repayment has occurred on a nonperforming loan to be reclassed to performing status.

+ References

No definition available.

+ Details

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- Definition

Policy for loan agreements to return to performing status whose terms have been modified at a below market rate because of the inability of the borrower for financial reasons to comply with the terms of the original loan agreement. A creditor discloses material facts pertaining to significant modifications.

+ References

No definition available.

+ Details

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- Definition

Minimum Assets Requirement

+ References

No definition available.

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- Definition

Policy for minimum period of time repayment has occurred on a restructured loan to be reclassed to performing status.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Policy for minimum period of time, in year subsequent to when impaired classification status was determined, that repayment has occurred on a TDR loan to be eligible for reclassification to performing status.

+ References

No definition available.

+ Details

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- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

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- Definition

The number of states the entity operates in as of the balance sheet date.

+ References

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- Definition

Describes the periods of time over which an entity anticipates to receive utility from its property, plant and equipment (that is, the periods of time over which an entity allocates the initial cost of its property, plant and equipment).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367646&loc=d3e18780-107790

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6539997&loc=d3e1205-110223

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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