Offsetting of Assets and Liabilities - Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	$ 1,947.0	$ 2,003.1
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,796.3	1,621.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	150.7	381.7
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0
Net Amount	150.7	381.7
Total Derivatives Not Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	441.4	825.9
Total Derivatives
GROSS RECOGNIZED LIABILITIES	2,388.4	2,829.0
Derivative Liabilities	592.1	1,207.6
Net Amount	592.1	1,207.6
Securities Sold under Agreements to Repurchase
GROSS RECOGNIZED LIABILITIES	168.3	834.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	168.3	834.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	(168.3)	(834.0)
Other liabilities	2,500.0	2,400.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	1,821.0	1,889.2
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,175.3	1,397.7
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	645.7	491.5
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0
Net Amount	645.7	491.5
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	68.8	69.2
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	19.0	14.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	49.8	55.0
Net Amount	49.8	55.0
Interest Rate Swaps | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	24.4	14.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	24.4	10.1
NET AMOUNTS PRESENTED IN THE BALANCE SHEET		4.2
Net Amount		4.2
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	32.8	30.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	32.8	30.4
Net Amount	32.8	30.4
Cross Product Netting Adjustment
Total Derivatives Subject to a Master Netting Arrangement
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3.9	10.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	$ 573.7	$ 189.0