Variable Interest Entities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Funding of asset's cost via an equity interest	20.00%
Funding of asset's cost via third party non recourse debt holders	80.00%
Carrying amounts	$ 768.2	$ 782.3
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	263.6	265.4
Liabilities related to unfunded commitments	$ 35.7	$ 35.5

Funding of asset's cost provided by third party non-recourse debt holders.

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Namespace Prefix:	ntrs
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Balance Type:	na
Period Type:	instant

- Definition

Funding of asset's cost via an equity ownership in a trust.

+ Details

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Namespace Prefix:	ntrs
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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.

+ Details

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Namespace Prefix:	us-gaap
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Balance Type:	na
Period Type:	duration

- Definition

The carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=6970148&loc=d3e5728-111685

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE).

+ References

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant