Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Net Amounts Presented	$ 919.6		$ 917.3
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	919.6		917.3
Net Amounts Presented	919.6		917.3
Gross Amounts Not Offset	919.6		917.3
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	1,983.3	[1]	3,100.9	[1]
Gross Amounts Offset	1,487.8	[1]	1,926.0	[1]
Net Amounts Presented	495.5	[1]	1,174.9	[1]
Net Amount	495.5	[1],[2]	1,174.9	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	1,645.5	[1]	2,234.2	[1]
Gross Amounts Offset	1,487.8	[1]	1,926.0	[1]
Net Amounts Presented	157.7	[1]	308.2	[1]
Net Amount	157.7	[1],[2]	308.2	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	337.8	[1]	866.7	[1]
Net Amounts Presented	337.8	[1]	866.7	[1]
Net Amount	337.8	[1],[2]	866.7	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	1,493.0	[1]	2,039.0	[1]
Gross Amounts Offset	790.2	[1]	1,073.3	[1]
Net Amounts Presented	702.8	[1]	965.7	[1]
Net Amount	702.8	[1],[2]	965.7	[1],[2]
Derivatives | Interest Rate Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	143.4	[1]	163.7	[1]
Gross Amounts Offset	38.6	[1]	47.5	[1]
Net Amounts Presented	104.8	[1]	116.2	[1]
Net Amount	104.8	[1],[2]	116.2	[1],[2]
Derivatives | Interest Rate Swap Contracts | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	9.1	[1]	31.5	[1]
Gross Amounts Offset	9.1	[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	68.8	[1]	28.4	[1]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 581.1	[1]	$ 767.7	[1]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,893.4 million and $2,338.4 million as of June 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 June 30, 2014 and December 31, 2013.