Offsetting of Assets and Liabilities - Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase(Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Other liabilities	$ 2,400.0	$ 2,700.0
Gross Recognized Liabilities	2,003.1	2,767.6
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	1,621.4	2,431.2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	381.7	336.4
Derivative Liability, Fair Value, Amount Offset Against Collateral	381.7	336.4
Derivative Liability, Not Subject to Master Netting Arrangement	825.9	608.5
Derivative Liability, Gross Recognized Liabilities	2,829.0	3,376.1
Derivative Liabilities	1,207.6	944.9
Securities Sold under Agreements to Repurchase, Gross	834.0	473.7
Balance at December 31	834.0	473.7
Gross Amounts Not Offset	(834.0)	(473.7)
Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	1,889.2	2,634.4
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	1,397.7	1,651.9
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	491.5	982.5
Derivative Liability, Fair Value, Amount Offset Against Collateral	491.5	982.5
Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	69.2	86.2
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	14.2	18.2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	55.0	68.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	55.0	68.0
Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	14.3	21.8
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	10.1	21.8
Other Financial Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	30.4	25.2
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	30.4	25.2
Derivative Liability, Fair Value, Amount Offset Against Collateral	30.4	25.2
Cross Product Netting Adjustment
Derivative [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	10.4	17.2
Cross Product Collateral Adjustment
Derivative [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ 189.0	$ 722.1