Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Net Amounts Presented		$ 29.8	$ 885.1
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities		29.8	885.1
Net Amounts Presented		29.8	885.1
Gross Amounts Not Offset		29.8	885.1
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	[1]	2,607.2	4,227.3
Gross Amounts Offset	[1]	1,461.8	3,173.3
Net Amounts Presented	[1]	1,145.4	1,054.0
Net Amount	[1],[2]	1,145.4	1,054.0
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	[1]	1,702.3	3,562.8
Gross Amounts Offset	[1]	1,461.8	3,173.3
Net Amounts Presented	[1]	240.5	389.5
Net Amount	[1],[2]	240.5	389.5
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	[1]	904.9	664.5
Net Amounts Presented	[1]	904.9	664.5
Net Amount	[1],[2]	904.9	664.5
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	[1]	1,570.5	3,431.0
Gross Amounts Offset	[1]	1,232.9	1,889.8
Net Amounts Presented	[1]	337.6	1,541.2
Net Amount	[1],[2]	337.6	1,541.2
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	[1]	101.3	118.7
Gross Amounts Offset	[1]	30.3	32.1
Net Amounts Presented	[1]	71.0	86.6
Net Amount	[1],[2]	71.0	86.6
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	[1]	19.2	13.1
Gross Amounts Offset	[1]	19.2	13.1
Derivatives | Other Financial Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	[1]	11.3
Net Amounts Presented	[1]	11.3
Net Amount	[1],[2]	11.3
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	[1]	27.7	6.3
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	[1]	$ 151.7	$ 1,232.0

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheets. Other liabilities (excluding derivative liabilities) totaled $2,519.5 million and $2,794.1 million as of June 30, 2015, and December 31, 2014, respectively.

[2] Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of June 30, 2015, and December 31, 2014.