Offsetting of Assets and Liabilities - Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	$ 3,738.0	$ 2,312.4
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2,817.1	1,618.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	920.9	694.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.5	0.1
Net Amount	920.4	693.9
Total Derivatives Not Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	1,144.8	1,000.6
Total Derivatives Not Subject to a Master Netting Arrangement	0.5	0.1
Total Derivatives
GROSS RECOGNIZED LIABILITIES	4,882.8	3,313.0
Derivative Liabilities	2,065.7	1,694.6
Net Amount	2,065.2	1,694.5
Securities Sold under Agreements to Repurchase
GROSS RECOGNIZED LIABILITIES	39.8	489.7
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	39.8	489.7
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	(39.8)	(489.7)
Other liabilities	3,500.0	3,100.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	3,577.7	2,181.6
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2,718.6	1,548.6
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	859.1	633.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.5	0.1
Net Amount	858.6	632.9
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	125.0	96.7
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	98.5	57.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	26.5	39.4
Net Amount	26.5	39.4
Interest Rate Swaps | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	0.0	0.7
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	0.0	0.7
Net Amount	0.0	0.7
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	35.3	20.9
Net Amount	35.3	20.9
Other Financial Derivatives | Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED LIABILITIES	35.3	33.4
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	$ 0.0	$ 12.5