Offsetting of Assets and Liabilities - Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	$ 2,312.4	$ 1,947.0
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,618.4	1,796.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	694.0	150.7
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.1
Net Amount	693.9	150.7
Total Derivatives Not Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	1,000.6	441.4
Total Derivatives Not Subject to a Master Netting Arrangement	0.1
Total Derivatives
GROSS RECOGNIZED LIABILITIES	3,313.0	2,388.4
Derivative Liabilities	1,694.6	592.1
Net Amount	1,694.5	592.1
Securities Sold under Agreements to Repurchase
GROSS RECOGNIZED LIABILITIES	489.7	168.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	489.7	168.3
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	(489.7)	(168.3)
Other liabilities	3,100.0	2,500.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	2,181.6	1,821.0
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,548.6	1,751.7
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	633.0	69.3
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.1
Net Amount	632.9	69.3
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	96.7	68.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	57.3	19.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	39.4	49.8
Net Amount	39.4	49.8
Interest Rate Swaps | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	0.7	24.4
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	24.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	0.7	0.0
Net Amount	0.7	0.0
Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Net Amount		31.6
Interest Rate Swaps Exchange Cleared | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	33.4	32.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	12.5	1.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	20.9	$ 31.6
Net Amount	$ 20.9