Components of Other Operating Expense (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Component of Operating Other Cost and Expense [Line Items]
Business Promotion	$ 88.0us-gaap_MarketingAndAdvertisingExpense	$ 91.6us-gaap_MarketingAndAdvertisingExpense	$ 87.8us-gaap_MarketingAndAdvertisingExpense
FDIC Insurance Premiums	22.0	23.5	25.4
Staff Related	39.1us-gaap_OtherLaborRelatedExpenses	39.1us-gaap_OtherLaborRelatedExpenses	41.9us-gaap_OtherLaborRelatedExpenses
Legal Settlement Charge		19.2
Other Expenses	103.5ntrs_NoninterestExpenseOther	119.7ntrs_NoninterestExpenseOther	107.5ntrs_NoninterestExpenseOther
Total Other Operating Expense	272.1us-gaap_OtherNoninterestExpense	314.2us-gaap_OtherNoninterestExpense	282.9us-gaap_OtherNoninterestExpense
Other Intangible Assets
Component of Operating Other Cost and Expense [Line Items]
Other Intangibles Amortization	$ 19.5	$ 21.1	$ 20.3

Noninterest Expense, Other

No definition available.

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- Definition

Pre-tax charge in connection with the agreement to resolve certain long-standing class action litigation related to Northern Trust Corporation's securities lending program.

+ References

No definition available.

+ Details

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

+ Details

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Balance Type:	debit
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- Details

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- Definition

Amount of expense for Federal Deposit Insurance Corporation (FDIC) insurance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_FederalDepositInsuranceCorporationPremiumExpense
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total expense recognized in the period for promotion, public relations, and brand or product advertising.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.4)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Amount of payroll tax expense, compensation, postemployment and benefit-related expenses not elsewhere specified in the taxonomy (such as health plan, profit sharing, incentives, tuition reimbursement, other fringe benefits and perquisites) incurred in the period.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Other noninterest expenses that are not separately presented in any other noninterest expense category.

+ References

+ Details

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- Details

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