Assets Measured at Fair Value on a Nonrecurring Basis (Detail) (Fair Value, Measurements, Nonrecurring, USD $) In Millions	Sep. 30, 2011		Sep. 30, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	$ 50.6		$ 82.0
Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	46.6	[1]	74.6	[1]
Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	4.0	[2]	7.4	[2]
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	50.6		82.0
Fair Value, Inputs, Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	46.6	[1]	74.6	[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	$ 4.0	[2]	$ 7.4	[2]

[1] Northern Trust provided an additional $3.0 million and $1.7 million of specific reserves to reduce the fair value of these loans during the three months ended September 30, 2011 and 2010, respectively. During the nine months ended September 30, 2011 and 2010, these loans were reduced by $10.5 million and $16.1 million, respectively.

[2] Northern Trust charged $1.7 million and $2.6 million through other operating expenses during the three months ended September 30, 2011 and 2010 respectively to reduce the fair values of these Other Real Estate Owned (OREO) properties. During the nine months ended September 30 2011 and 2010, the fair values of these OREO properties were reduced by $1.8 million and $3.8 million, respectively.