Assets Measured at Fair Value on Nonrecurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	$ 33.9		$ 26.9
Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	32.4	[1]	25.5	[1]
Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	1.5	[2]	1.4	[2]
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	33.9		26.9
Fair Value, Inputs, Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Asset	32.4	[1]	25.5	[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	$ 1.5	[2]	$ 1.4	[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 months ended March 31, 2013 increased by $1.9 million the level of specific allowances on these loans. During the three months ended March 31, 2012, Northern Trust reduced by $9.5 million the level of specific allowances on 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 $0.3 million and $0.1 million through other operating expenses during the three months ended March 31, 2013 and March 31, 2012, respectively, to reduce the fair values of these OREO properties.