Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Derivative [Line Items]
Net Amounts Presented	$ 613.2		$ 885.1
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	613.2		885.1
Net Amounts Presented	613.2		885.1
Gross Amounts Not Offset	613.2		885.1
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	4,862.3	[1]	4,227.3	[1]
Gross Amounts Offset	3,510.9	[1]	3,173.3	[1]
Net Amounts Presented	1,351.4	[1]	1,054.0	[1]
Net Amount	1,351.4	[1],[2]	1,054.0	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	4,043.3	[1]	3,562.8	[1]
Gross Amounts Offset	3,510.9	[1]	3,173.3	[1]
Net Amounts Presented	532.4	[1]	389.5	[1]
Net Amount	532.4	[1],[2]	389.5	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	819.0	[1]	664.5	[1]
Net Amounts Presented	819.0	[1]	664.5	[1]
Net Amount	819.0	[1],[2]	664.5	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	3,895.1	[1]	3,431.0	[1]
Gross Amounts Offset	2,360.5	[1]	1,889.8	[1]
Net Amounts Presented	1,534.6	[1]	1,541.2	[1]
Net Amount	1,534.6	[1],[2]	1,541.2	[1],[2]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	122.5	[1]	118.7	[1]
Gross Amounts Offset	31.8	[1]	32.1	[1]
Net Amounts Presented	90.7	[1]	86.6	[1]
Net Amount	90.7	[1],[2]	86.6	[1],[2]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	25.7	[1]	13.1	[1]
Gross Amounts Offset	25.7	[1]	13.1	[1]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	25.4	[1]	6.3	[1]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 1,067.5	[1]	$ 1,232.0	[1]

[1] Derivative liabilities are reported in other liabilities in the consolidated balance sheets. Other liabilities (excluding derivative liabilities) totaled $3,185.1 million and $2,794.1 million as of March 31, 2015, and December 31, 2014, respectively.

[2] Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.