Contingent Liabilities - Additional Information (Detail) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit outstanding	$ 4,400,000,000	$ 4,300,000,000
Standby letters of credit, unamortized fees	46,700,000	36,700,000
Minimum securities lending related collateral percentage	100.00%	100.00%
Possible loss in future periods in excess of accrual	31,000,000
Indemnification Agreement
Commitments and Contingencies Disclosure [Line Items]
Collateralized securities loaned, subject to indemnification	78,100,000,000	74,900,000,000
Credit loss liability	0	0
Visa Indemnification
Commitments and Contingencies Disclosure [Line Items]
Fair value of net Visa indemnification liability	$ 13,000,000	$ 23,100,000

Commitments and Contingencies Disclosure [Line Items]

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- Definition

Exposure to indemnifications on credit losses from securities custody activities as of balance sheet date.

+ Details

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- Definition

Carrying amount as of balance sheet date of the Visa related indemnification liability, included within Other Liabilities.

+ Details

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- Definition

Minimum percentage of collateral maintained for client securities loaned at the fair value of securities plus interest.

+ Details

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- Definition

Amount of client securities loaned that are subject to indemnification as of the balance sheet date.

+ Details

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- Definition

Amount of unamortized fees on standby letters of credit reflecting Northern Trust's recorded liability for these instruments as of the balance sheet date.

+ Details

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ Details

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- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

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

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 14

-Paragraph 3

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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