Offsetting of Assets and Liabilities - Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Gross Recognized Assets	$ 2,203.3	$ 3,047.6
Gross Amounts Offset	1,860.0	2,170.4
Net Amounts Presented Not Subject To Master Arrangements	343.3	877.2
Net Amount Not Subject To Master Arrangements	343.3	877.2
Total Derivatives Not Subject to a Master Netting Arrangement	450.8	809.2
Total Derivatives, Gross	2,654.1	3,856.8
Derivative Asset	794.1	1,686.4
Securities Purchased under Agreements to Resell, Gross	1,303.3	1,967.5
Securities Purchased under Agreements to Resell	1,303.3	1,967.5
Securities Purchased Under Agreements To Resell, Gross Amounts Not Offset	1,303.3	1,967.5
Other Assets (excluding Derivative Assets)	3,900.0	3,300.0
Federal Funds Sold	21.0	6.8
Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	2,106.3	2,800.4
Gross Amounts Offset	1,397.7	1,651.9
Net Amounts Presented Not Subject To Master Arrangements	708.6	1,148.5
Net Amount Not Subject To Master Arrangements	708.6	1,148.5
Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	86.9	129.8
Gross Amounts Offset	14.2	18.2
Net Amounts Presented Not Subject To Master Arrangements	72.7	111.6
Net Amount Not Subject To Master Arrangements	72.7	111.6
Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	10.1	117.4
Gross Amounts Offset	10.1	21.8
Net Amounts Presented Not Subject To Master Arrangements	0.0	95.6
Net Amount Not Subject To Master Arrangements	0.0	95.6
Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	10.4	17.2
Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$ 427.6	$ 461.3