Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	$ 1,000.0	[1]	$ 500.0	[1]
Net Amount Presented	1,000.0	[1]	500.0	[1]
Gross Amounts Not Offset	1,000.0	[1]	500.0	[1]
Derivatives
Derivative [Line Items]
Gross Recognized Assets	4,417.3	[2]	3,103.6	[2]
Gross Amounts Offset	2,358.4	[2]	1,369.0	[2]
Net Amount Presented	2,058.9	[2]	1,734.6	[2]
Net Amount	2,058.9	[2],[3]	1,734.6	[2],[3]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	3,478.5	[2]	2,850.4	[2]
Gross Amounts Offset	2,358.4	[2]	1,369.0	[2]
Net Amount Presented	1,120.1	[2]	1,481.4	[2]
Net Amount	1,120.1	[2],[3]	1,481.4	[2],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	938.8	[2]	253.2	[2]
Net Amount Presented	938.8	[2]	253.2	[2]
Net Amount	938.8	[2],[3]	253.2	[2],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	3,265.0	[2]	2,612.5	[2]
Gross Amounts Offset	2,085.0	[2]	1,073.3	[2]
Net Amount Presented	1,180.0	[2]	1,539.2	[2]
Net Amount	1,180.0	[2],[3]	1,539.2	[2],[3]
Derivatives | Interest Rate Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	183.7	[2]	228.8	[2]
Gross Amounts Offset	31.5	[2]	47.5	[2]
Net Amount Presented	152.2	[2]	181.3	[2]
Net Amount	152.2	[2],[3]	181.3	[2],[3]
Derivatives | Interest Rate Swap Contracts | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	29.8	[2]	9.1	[2]
Gross Amounts Offset	7.4	[2]	9.1	[2]
Net Amount Presented	22.4	[2]
Net Amount	22.4	[2],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	27.1	[2]	28.4	[2]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 207.4	[2]	$ 210.7	[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 sheet. Federal funds sold totaled $10.0 million and $29.6 million as of September 30, 2014, and December 31, 2013, respectively.

[2] Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $3,230.9 million and $3,029.4 million as of September 30, 2014, and December 31, 2013, respectively.

[3] Northern Trust did not possess any cash collateral 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 September 30, 2014, and December 31, 2013.