Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Net Amounts Presented	$ 866.8		$ 917.3
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	866.8		917.3
Net Amounts Presented	866.8		917.3
Gross Amounts Not Offset	866.8		917.3
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	4,265.1	[1]	3,100.9	[1]
Gross Amounts Offset	2,752.3	[1]	1,926.0	[1]
Net Amounts Presented	1,512.8	[1]	1,174.9	[1]
Net Amount	1,512.8	[1],[2]	1,174.9	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	3,527.8	[1]	2,234.2	[1]
Gross Amounts Offset	2,752.3	[1]	1,926.0	[1]
Net Amounts Presented	775.5	[1]	308.2	[1]
Net Amount	775.5	[1],[2]	308.2	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	737.3	[1]	866.7	[1]
Net Amounts Presented	737.3	[1]	866.7	[1]
Net Amount	737.3	[1],[2]	866.7	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	3,404.3	[1]	2,039.0	[1]
Gross Amounts Offset	2,085.0	[1]	1,073.3	[1]
Net Amounts Presented	1,319.3	[1]	965.7	[1]
Net Amount	1,319.3	[1],[2]	965.7	[1],[2]
Derivatives | Interest Rate Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	116.1	[1]	163.7	[1]
Gross Amounts Offset	31.5	[1]	47.5	[1]
Net Amounts Presented	84.6	[1]	116.2	[1]
Net Amount	84.6	[1],[2]	116.2	[1],[2]
Derivatives | Interest Rate Swap Contracts | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	7.4	[1]	31.5	[1]
Gross Amounts Offset	7.4	[1]	9.1	[1]
Net Amounts Presented			22.4	[1]
Net Amount			22.4	[1],[2]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	27.1	[1]	28.4	[1]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 601.3	[1]	$ 767.7	[1]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,591.5 million and $2,338.4 million as of September 30, 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 September 30, 2014, and December 31, 2013.