Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$ 30,906.8		$ 30,192.5
Trading Account	9.3		8.0
Contingent Consideration	58.6		56.8
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading	30,916.1		30,200.5
Standby Letters of Credit	50.1		43.8
Contingent Consideration	58.6		56.8
Total Other	108.7		100.6
Fair Value, Measurements, Recurring | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	30,906.8		30,192.5
Trading Account	9.3		8.0
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,766.5		4,029.4
Fair Value, Measurements, Recurring | Securities Investment | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	15.0		15.8
Fair Value, Measurements, Recurring | Securities Investment | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17,605.3		16,771.4
Fair Value, Measurements, Recurring | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,839.7		2,676.7
Fair Value, Measurements, Recurring | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,376.4		754.9
Fair Value, Measurements, Recurring | Securities Investment | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	192.2		173.7
Fair Value, Measurements, Recurring | Securities Investment | Supranational Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	911.1		972.1
Fair Value, Measurements, Recurring | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	137.6		163.8
Fair Value, Measurements, Recurring | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,035.0		1,604.8
Fair Value, Measurements, Recurring | Securities Investment | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,448.5		2,418.1
Fair Value, Measurements, Recurring | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	176.3		178.3
Fair Value, Measurements, Recurring | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	403.2		433.5
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	1,297.5		1,182.6
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	3,142.4		3,087.3
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	323.9		338.3
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Option
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	0.1
Fair Value, Measurements, Recurring | Other Assets | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets			0.7
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	1,220.7		942.6
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	3,109.2		2,991.6
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	222.9		231.9
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Option
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	0.1
Fair Value, Measurements, Recurring | Other Liabilities | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	0.2		0.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading	2,766.5		4,029.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,766.5		4,029.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,766.5		4,029.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading	27,973.3		25,992.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	27,964.0		25,984.8
Trading Account	9.3		8.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	15.0		15.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17,605.3		16,771.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,839.7		2,676.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,376.4		754.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	192.2		173.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	911.1		972.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	137.6		163.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,035.0		1,604.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,448.5		2,418.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	403.2		433.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	3,466.4		3,426.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	3,142.4		3,087.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Interest Rate Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	323.9		338.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Interest Rate Option
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	0.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets			0.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	3,332.4		3,223.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	3,109.2		2,991.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Interest Rate Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	222.9		231.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Interest Rate Option
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	0.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	0.2		0.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading	176.3		178.3
Standby Letters of Credit	50.1		43.8
Contingent Consideration	58.6		56.8
Total Other	108.7		100.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	176.3		178.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	176.3		178.3
Fair Value, Measurements, Recurring | Netting and Collateral | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Assets	(2,168.9)	[1]	(2,243.7)	[2]
Fair Value, Measurements, Recurring | Netting and Collateral | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivatives Liabilities	$ (2,111.7)	[1]	$ (2,281.0)	[2]

[1] Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting agreements exist between Northern Trust and the counterparty. As of March 31, 2012, derivative assets and liabilities shown above also include reductions of $350.3 million and $293.1 million, respectively, as a result of cash collateral received from and deposited with derivative counterparties.

[2] Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting agreements exist between Northern Trust and the counterparty. As of December 31, 2011, derivative assets and liabilities shown above also include reductions of $220.1 million and $257.4 million, respectively, as a result of cash collateral received from and deposited with derivative counterparties.