Assets Measured at Fair Value on Nonrecurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Sep. 30, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	$ 28.7		$ 50.6
Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	27.7	[1]	46.6	[1]
Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	1.0	[2]	4.0	[2]
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	28.7		50.6
Fair Value, Inputs, Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	27.7	[1]	46.6	[1]
Fair Value, Inputs, Level 3 | Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	$ 1.0	[2]	$ 4.0	[2]

[1] In accordance with Accounting Standard Codification (ASC) Subtopic 310-10, Northern Trust recorded individually impaired loans at fair value and, for the three and nine months ended September 30, 2012, respectively, increased by $5.7 million and reduced by $8.5 million the level of specific allowances on these loans. During the three and nine months ended September 30, 2011, Northern Trust provided an additional $3.0 million and $10.5 million, respectively, of specific allowances to reduce the fair value of these loans.

[2] In accordance with ASC Subtopics 310-40 and 360-10, Northern Trust recorded Other Real Estate Owned (OREO) at fair value and subsequently charged $0.2 million and $0.4 million through other operating expenses during the three and nine months ended September 30, 2012, respectively, to reduce the fair values of these OREO properties. Charges of $1.7 million and $1.8 million were recorded through other operating expenses during the three and nine months ended September 30, 2011, respectively, to reduce the fair values of these OREO properties.