Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	$ 2,519.3	[1],[2]	$ 2,023.0	[1],[3]
Gross Amounts Offset	1,979.9	[1],[2]	1,407.5	[1],[3]
Net Amount Presented	539.4	[1],[2]	615.5	[1],[3]
Gross Amounts Not Offset		[1],[2]		[1],[3]
Net Amount	539.4	[1],[2],[4]	615.5	[1],[3],[5]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	2,298.4	[2]	1,772.7	[3]
Gross Amounts Offset	1,411.5	[2]	877.1	[3]
Net Amount Presented	886.9	[2]	895.6	[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount	886.9	[2],[4]	895.6	[3],[5]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	220.8	[2]	249.3	[3]
Gross Amounts Offset	60.2	[2]	68.3	[3]
Net Amount Presented	160.6	[2]	181.0	[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount	160.6	[2],[4]	181.0	[3],[5]
Derivatives | Credit Default Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	0.1	[2]	1.0	[3]
Gross Amounts Offset		[2]		[3]
Net Amount Presented	0.1	[2]	1.0	[3]
Gross Amounts Not Offset		[2]		[3]
Net Amount	0.1	[2],[4]	1.0	[3],[5]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[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 Liabilities		[2]		[3]
Gross Amounts Offset	472.2	[2]	425.0	[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 Liabilities	490.2		699.8
Gross Amounts Offset
Net Amount Presented	490.2		699.8
Gross Amounts Not Offset	490.2		699.8
Net Amount		[4]		[5]

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

[2] Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,059.7 million as of March 31, 2013.

[3] 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.

[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 March 31, 2013.

[5] 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.