Offsetting of Assets and Liabilities - Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Total Derivatives Subject to a Master Netting Arrangement
Assets	$ 4,097.2	$ 2,844.0
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3,507.8	2,338.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	589.4	506.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.9	16.5
Net Amount	588.5	489.5
Total Derivatives Not Subject to a Master Netting Arrangement
Total Derivatives Not Subject to a Master Netting Arrangement	461.0	543.7
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0	0.3
Net Amount	461.0	543.4
Total Derivatives
GROSS RECOGNIZED ASSETS	4,558.2	3,387.7
Derivative Assets	1,050.4	1,049.7
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.9	16.8
Net Amount	1,049.5	1,032.9
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	1,596.5	707.8
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,596.5	707.8
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	1,596.5	707.8
Other assets (excluding derivative assets)	7,300.0	7,400.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Assets	3,799.7	2,691.1
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3,505.3	2,334.1
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	294.4	357.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.9	16.5
Net Amount	293.5	340.5
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Assets	295.9	151.9
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2.5	3.9
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	293.4	148.0
Net Amount	293.4	148.0
Interest Rate Swaps | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Assets	1.6	1.0
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1.6	1.0
Net Amount	$ 1.6	$ 1.0