Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheets (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	[1]	$ 1,000.0	$ 1,000.0
Net Amounts Presented	[1]	1,000.0	1,000.0
Gross Amounts Not Offset	[1]	1,000.0	1,000.0
Derivatives
Derivative [Line Items]
Gross Recognized Assets	[2]	2,686.3	4,507.5
Gross Amounts Offset	[2]	1,698.0	2,257.1
Net Amounts Presented	[2]	988.3	2,250.4
Net Amount	[2],[3]	988.3	2,250.4
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	[2]	2,297.6	3,675.1
Gross Amounts Offset	[2]	1,698.0	2,257.1
Net Amounts Presented	[2]	599.6	1,418.0
Net Amount	[2],[3]	599.6	1,418.0
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	[2]	388.7	832.4
Net Amounts Presented	[2]	388.7	832.4
Net Amount	[2],[3]	388.7	832.4
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	[2]	2,093.4	3,442.8
Gross Amounts Offset	[2]	1,232.9	1,889.8
Net Amounts Presented	[2]	860.5	1,553.0
Net Amount	[2],[3]	860.5	1,553.0
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	[2]	167.8	183.9
Gross Amounts Offset	[2]	30.3	32.1
Net Amounts Presented	[2]	137.5	151.8
Net Amount	[2],[3]	137.5	151.8
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	[2]	36.4	48.4
Gross Amounts Offset	[2]	19.2	13.1
Net Amounts Presented	[2]	17.2	35.3
Net Amount	[2],[3]	17.2	35.3
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	[2]	27.7	6.3
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	[2]	$ 387.9	$ 315.8

[1] Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheets. Federal funds sold totaled $20.0 million and $62.7 million as of June 30, 2015, and December 31, 2014, respectively.

[2] Derivative assets are reported in other assets in the consolidated balance sheets. Other assets (excluding derivative assets) totaled $3,113.6 million and $3,614.7 million as of June 30, 2015, and December 31, 2014, respectively.

[3] Northern Trust did not possess any cash collateral 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.