Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	$ 1,000.0	[1]	$ 1,000.0	[1]
Net Amounts Presented	1,000.0	[1]	1,000.0	[1]
Gross Amounts Not Offset	1,000.0	[1]	1,000.0	[1]
Derivatives
Derivative [Line Items]
Gross Recognized Assets	5,070.6	[2]	4,507.5	[2]
Gross Amounts Offset	2,884.2	[2]	2,257.1	[2]
Net Amounts Presented	2,186.4	[2]	2,250.4	[2]
Net Amount	2,186.4	[2],[3]	2,250.4	[2],[3]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	3,977.1	[2]	3,675.1	[2]
Gross Amounts Offset	2,884.2	[2]	2,257.1	[2]
Net Amounts Presented	1,092.9	[2]	1,418.0	[2]
Net Amount	1,092.9	[2],[3]	1,418.0	[2],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	1,093.5	[2]	832.4	[2]
Net Amounts Presented	1,093.5	[2]	832.4	[2]
Net Amount	1,093.5	[2],[3]	832.4	[2],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	3,705.1	[2]	3,442.8	[2]
Gross Amounts Offset	2,360.5	[2]	1,889.8	[2]
Net Amounts Presented	1,344.6	[2]	1,553.0	[2]
Net Amount	1,344.6	[2],[3]	1,553.0	[2],[3]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	197.9	[2]	183.9	[2]
Gross Amounts Offset	31.8	[2]	32.1	[2]
Net Amounts Presented	166.1	[2]	151.8	[2]
Net Amount	166.1	[2],[3]	151.8	[2],[3]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	74.1	[2]	48.4	[2]
Gross Amounts Offset	25.7	[2]	13.1	[2]
Net Amounts Presented	48.4	[2]	35.3	[2]
Net Amount	48.4	[2],[3]	35.3	[2],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	25.4	[2]	6.3	[2]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 440.8	[2]	$ 315.8	[2]

[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 $113.9 million and $62.7 million as of March 31, 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 $3,177.3 million and $3,614.7 million as of March 31, 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 March 31, 2015, and December 31, 2014.