Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	$ 500.0	[1]	$ 500.0	[1]
Net Amount Presented	500.0	[1]	500.0	[1]
Gross Amounts Not Offset	500.0	[1]	500.0	[1]
Derivatives
Derivative [Line Items]
Gross Recognized Assets	2,020.6	[2]	3,103.6	[2]
Gross Amounts Offset	1,458.7	[2]	1,369.0	[2]
Net Amount Presented	561.9	[2]	1,734.6	[2]
Net Amount	561.9	[2],[3]	1,734.6	[2],[3]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	1,737.1	[2]	2,850.4	[2]
Gross Amounts Offset	1,458.7	[2]	1,369.0	[2]
Net Amount Presented	278.4	[2]	1,481.4	[2]
Net Amount	278.4	[2],[3]	1,481.4	[2],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	283.5	[2]	253.2	[2]
Net Amount Presented	283.5	[2]	253.2	[2]
Net Amount	283.5	[2],[3]	253.2	[2],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	1,513.4	[2]	2,612.5	[2]
Gross Amounts Offset	790.2	[2]	1,073.3	[2]
Net Amount Presented	723.2	[2]	1,539.2	[2]
Net Amount	723.2	[2],[3]	1,539.2	[2],[3]
Derivatives | Interest Rate Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	205.1	[2]	228.8	[2]
Gross Amounts Offset	38.6	[2]	47.5	[2]
Net Amount Presented	166.5	[2]	181.3	[2]
Net Amount	166.5	[2],[3]	181.3	[2],[3]
Derivatives | Interest Rate Swap Contracts | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	18.6	[2]	9.1	[2]
Gross Amounts Offset	9.1	[2]	9.1	[2]
Net Amount Presented	9.5	[2]
Net Amount	9.5	[2],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	68.8	[2]	28.4	[2]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 552.0	[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 $79.5 million and $29.6 million as of June 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 $3,308.3 million and $3,029.4 million as of June 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 June 30, 2014 and December 31, 2013.