Assets Measured at Fair Value on Nonrecurring Basis (Detail) (Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Jun. 30, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	$ 24.3		$ 64.9
Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	23.2	[1]	62.2	[1]
Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	1.1	[2]	2.7	[2]
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	24.3		64.9
Fair Value, Inputs, Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	23.2	[1]	62.2	[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	$ 1.1	[2]	$ 2.7	[2]

[1] In accordance with Accounting Standard Codification (ASC) Subtopic 310-10, Northern Trust recorded individually impaired loans at fair value and reduced by $4.7 million and $14.2 million the level of specific allowances on these loans during the three and six months ended June 30, 2012, respectively. An additional $5.2 million and $4.7 million of specific allowances were provided during the three and six months ended June 30, 2011, respectively, 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 $0.03 million and $0.1 million through other operating expenses during the three and six months ended June 30, 2012, respectively, to reduce the fair values of these OREO properties. Charges of $0.8 million and $0.9 million were recorded through other operating expenses during the three and six months ended June 30, 2011, respectively, to reduce the fair values of these OREO properties.