Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	$ 4,789.3	[1]	$ 2,023.0	[2]
Gross Amounts Offset	3,680.3	[1]	1,407.5	[2]
Net Amount Presented	1,109.0	[1]	615.5	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	1,109.0	[1]	615.5	[2],[3]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	4,407.9	[1]
Gross Amounts Offset	3,680.3	[1]
Net Amount Presented	727.6	[1]
Gross Amounts Not Offset		[1]
Net Amount	727.6	[1]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	381.4	[1]
Gross Amounts Offset		[1]
Net Amount Presented	381.4	[1]
Gross Amounts Not Offset		[1]
Net Amount	381.4	[1]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	4,240.1	[4]	1,772.7	[2]
Gross Amounts Offset	1,403.1	[4]	877.1	[2]
Net Amount Presented	2,837.0	[4]	895.6	[2]
Gross Amounts Not Offset		[4]		[2]
Net Amount	2,837.0	[1],[4]	895.6	[2],[3]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	167.1	[1]	249.3	[2]
Gross Amounts Offset	60.9	[1]	68.3	[2]
Net Amount Presented	106.2	[1]	181.0	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	106.2	[1]	181.0	[2],[3]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	0.5	[1]
Gross Amounts Offset	0.5	[1]
Net Amount Presented		[1]
Gross Amounts Not Offset		[1]
Net Amount		[1]
Derivatives | Interest Rate Option | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	0.1	[1]
Gross Amounts Offset		[1]
Net Amount Presented	0.1	[1]
Gross Amounts Not Offset		[1]
Net Amount	0.1	[1]
Derivatives | Credit Default Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	0.1	[1]	1.0	[2]
Gross Amounts Offset		[1]		[2]
Net Amount Presented	0.1	[1]	1.0	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	0.1	[1]	1.0	[2],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[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]		[2],[3]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[1]		[2]
Gross Amounts Offset	2,193.2	[1]	425.0	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1]		[2],[3]
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	777.7		699.8
Gross Amounts Offset
Net Amount Presented	777.7		699.8
Gross Amounts Not Offset	777.7		699.8
Net Amount		[1]		[3]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $3,201.0 million as of June 30, 2013.

[2] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $1,961.7 million as of December 31, 2012.

[3] Northern Trust did not place any cash collateral with counterparties 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.

[4] Northern Trust did not place any cash collateral with counterparties 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.