Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivatives
Derivative [Line Items]
Gross Recognized Assets	$ 4,941.9	[1]	$ 2,066.9	[2]
Gross Amounts Offset	1,555.9	[1]	1,101.1	[2]
Net Amount Presented	3,386.0	[1]	965.8	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	3,386.0	[1],[3]	965.8	[2],[4]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	4,529.6	[1]
Gross Amounts Offset	1,555.9	[1]
Net Amount Presented	2,973.7	[1]
Gross Amounts Not Offset		[1]
Net Amount	2,973.7	[1],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	412.3	[1]
Gross Amounts Offset		[1]
Net Amount Presented	412.3	[1]
Gross Amounts Not Offset		[1]
Net Amount	412.3	[1],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	4,403.4	[1]	1,756.6	[2]
Gross Amounts Offset	1,403.1	[1]	877.1	[2]
Net Amount Presented	3,000.3	[1]	879.5	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	3,000.3	[1],[3]	879.5	[2],[4]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	120.3	[1]	310.3	[2]
Gross Amounts Offset	60.9	[1]	68.3	[2]
Net Amount Presented	59.4	[1]	242.0	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	59.4	[1],[3]	242.0	[2],[4]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	5.8	[1]
Gross Amounts Offset	0.5	[1]
Net Amount Presented	5.3	[1]
Net Amount	5.3	[1],[3]
Derivatives | Interest Rate Option | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	0.1	[1]
Gross Amounts Offset		[1]
Net Amount Presented	0.1	[1]
Gross Amounts Not Offset		[1]
Net Amount	0.1	[1],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Assets		[1]		[2]
Gross Amounts Offset	22.6	[1]	37.1	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Assets		[1]		[2]
Gross Amounts Offset	68.8	[1]	118.6	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	500.0	[5]	35.4	[6]
Gross Amounts Offset		[5]		[6]
Net Amount Presented	500.0	[5]	35.4	[6]
Gross Amounts Not Offset	500.0	[5]	35.4	[6]
Net Amount		[3],[5]		[4],[6]

[1] Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $2,839.0 million as of June 30, 2013.

[2] 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.

[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, 2013.

[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 December 31, 2012.

[5] 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 $31.2 million as of June 30, 2013.

[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 $25.4 million as of December 31, 2012.