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, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$ 30,945.7		$ 29,558.5
Trading Account	1.4		4.7
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	30,947.1		29,563.2
Fair Value, Measurements, Recurring | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	30,945.7		29,558.5
Trading Account	1.4		4.7
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4,627.4		4,506.9
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	4.5		4.6
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	16,634.2		16,389.2
Fair Value, Measurements, Recurring | Securities Investment | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	311.4		310.4
Fair Value, Measurements, Recurring | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,651.3		3,577.7
Fair Value, Measurements, Recurring | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,049.3		1,907.5
Fair Value, Measurements, Recurring | Securities Investment | Sub-Sovereign, Supranational and Non US Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	561.0
Fair Value, Measurements, Recurring | Securities Investment | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			360.6
Fair Value, Measurements, Recurring | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4.4		6.4
Fair Value, Measurements, Recurring | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,949.3		2,321.3
Fair Value, Measurements, Recurring | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17.5		18.1
Fair Value, Measurements, Recurring | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	135.4		155.8
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	2,186.4		2,250.4
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	4,798.6		4,275.2
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	272.0		232.3
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,351.4		1,054.0
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	4,714.1		4,095.5
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	148.2		131.8
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 Securities	4,627.4		4,506.9
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	4,627.4		4,506.9
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	4,627.4		4,506.9
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 Securities	26,302.2		25,038.2
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	26,300.8		25,033.5
Trading Account	1.4		4.7
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	4.5		4.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | U.S. Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	16,634.2		16,389.2
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	311.4		310.4
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	3,651.3		3,577.7
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	2,049.3		1,907.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Sub-Sovereign, Supranational and Non US Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	561.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			360.6
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	4.4		6.4
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	2,949.3		2,321.3
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	135.4		155.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	5,070.6		4,507.5
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]
Derivative Assets	4,798.6		4,275.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	272.0		232.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	4,862.3		4,227.3
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]
Derivative Liabilities	4,714.1		4,095.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	148.2		131.8
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 Securities	17.5		18.1
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	17.5		18.1
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	17.5		18.1
Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	(2,884.2)	[1]	(2,257.1)	[2]
Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	$ (3,510.9)	[1]	$ (3,173.3)	[2]

[1] Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of March 31, 2015, derivative assets and liabilities shown above also include reductions of $440.8 million and $1.1 billion, 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 arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2014, derivative assets and liabilities shown above also include reductions of $315.8 million and $1.2 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.