Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Net Amounts Presented	$ 885.1		$ 917.3
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	885.1		917.3
Net Amounts Presented	885.1		917.3
Gross Amounts Not Offset	885.1		917.3
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	4,227.3	[1]	3,100.9	[1]
Gross Amounts Offset	3,173.3	[1]	1,926.0	[1]
Net Amounts Presented	1,054.0	[1]	1,174.9	[1]
Net Amount	1,054.0	[1],[2]	1,174.9	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	3,562.8	[1]	2,234.2	[1]
Gross Amounts Offset	3,173.3	[1]	1,926.0	[1]
Net Amounts Presented	389.5	[1]	308.2	[1]
Net Amount	389.5	[1],[2]	308.2	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	664.5	[1]	866.7	[1]
Net Amounts Presented	664.5	[1]	866.7	[1]
Net Amount	664.5	[1],[2]	866.7	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	3,431.0	[1]	2,039.0	[1]
Gross Amounts Offset	1,889.8	[1]	1,073.3	[1]
Net Amounts Presented	1,541.2	[1]	965.7	[1]
Net Amount	1,541.2	[1],[2]	965.7	[1],[2]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	118.7	[1]	163.7	[1]
Gross Amounts Offset	32.1	[1]	47.5	[1]
Net Amounts Presented	86.6	[1]	116.2	[1]
Net Amount	86.6	[1],[2]	116.2	[1],[2]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	13.1	[1]	31.5	[1]
Gross Amounts Offset	13.1	[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	6.3	[1]	28.4	[1]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 1,232.0	[1]	$ 767.7	[1]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,794.1 million and $2,338.4 million as of December 31, 2014 and 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 December 31, 2014 and 2013.