Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Net Amounts Presented	$ 736.6		$ 917.3
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	2,045.2	[1]	3,100.9	[1]
Gross Amounts Offset	1,445.0	[1]	1,926.0	[1]
Net Amounts Presented	600.2	[1]	1,174.9	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount	600.2	[1],[2]	1,174.9	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	1,677.1	[1]	2,234.2	[1]
Gross Amounts Offset	1,445.0	[1]	1,926.0	[1]
Net Amounts Presented	232.1	[1]	308.2	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount	232.1	[1],[2]	308.2	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	368.1	[1]	866.7	[1]
Gross Amounts Offset		[1]		[1]
Net Amounts Presented	368.1	[1]	866.7	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount	368.1	[1],[2]	866.7	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	1,522.3	[1]	2,039.0	[1]
Gross Amounts Offset	965.7	[1]	1,073.3	[1]
Net Amounts Presented	556.6	[1]	965.7	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount	556.6	[1],[2]	965.7	[1],[2]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	149.1	[1]	163.7	[1]
Gross Amounts Offset	41.7	[1]	47.5	[1]
Net Amounts Presented	107.4	[1]	116.2	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount	107.4	[1],[2]	116.2	[1],[2]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	5.7	[1]	31.5	[1]
Gross Amounts Offset	5.7	[1]	9.1	[1]
Net Amounts Presented		[1]	22.4	[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount		[1],[2]	22.4	[1],[2]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[1]		[1]
Gross Amounts Offset	67.0	[1]	28.4	[1]
Net Amounts Presented		[1]		[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount		[1],[2]		[1],[2]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[1]		[1]
Gross Amounts Offset	364.9	[1]	767.7	[1]
Net Amounts Presented		[1]		[1]
Gross Amounts Not Offset		[1]		[1]
Net Amount		[1],[2]		[1],[2]
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	736.6		917.3
Gross Amounts Offset
Net Amounts Presented	736.6		917.3
Gross Amounts Not Offset	736.6		917.3
Net Amount		[2]		[2]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,608.3 million and $2,338.4 million as of March 31, 2014 and December 31, 2013, respectively.

[2] Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of March 31, 2014 and December 31, 2013.