Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives
Derivative [Line Items]
Gross Recognized Assets	$ 2,674.0	[1],[2]	$ 2,066.9	[1],[3]
Gross Amounts Offset	1,725.8	[1],[2]	1,101.1	[1],[3]
Net Amount Presented	948.2	[1],[2]	965.8	[1],[3]
Gross Amounts Not Offset		[1],[2]		[1],[3]
Net Amount	948.2	[1],[2],[4]	965.8	[1],[3],[5]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	2,390.5	[2]	1,756.6	[3]
Gross Amounts Offset	1,411.5	[2]	877.1	[3]
Net Amount Presented	979.0	[2]	879.5	[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount	979.0	[2],[4]	879.5	[3],[5]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	283.5	[2]	310.3	[3]
Gross Amounts Offset	60.2	[2]	68.3	[3]
Net Amount Presented	223.3	[2]	242.0	[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount	223.3	[2],[4]	242.0	[3],[5]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Assets		[2]		[3]
Gross Amounts Offset	36.0	[2]	37.1	[3]
Net Amount Presented		[2]		[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount		[2],[4]		[3],[5]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Assets		[2]		[3]
Gross Amounts Offset	218.1	[2]	118.6	[3]
Net Amount Presented		[2]		[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount		[2],[4]		[3],[5]
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	250.0	[6]	35.4	[7]
Gross Amounts Offset		[6]		[7]
Net Amount Presented	250.0	[6]	35.4	[7]
Gross Amounts Not Offset	250.0	[6]	35.4	[7]
Net Amount		[4],[6]		[5],[7]

[1] All derivative assets are subject to a master netting or similar arrangement.

[2] Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $3,177.7 million as of March 31, 2013.

[3] Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $2,964.4 million as of December 31, 2012.

[4] 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 March 31, 2013.

[5] 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 December 31, 2012.

[6] 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 $1.5 million as of March 31, 2013.

[7] 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 $25.4 million as of December 31, 2012.