Offsetting of Assets and Liabilities - Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	$ 2,844.0	$ 1,999.7
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2,338.0	1,357.1
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	506.0	642.6
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	16.5	12.7
Net Amount	489.5	629.9
Total Derivatives Not Subject to a Master Netting Arrangement
Total Derivatives Not Subject to a Master Netting Arrangement	543.7	563.8
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.3	2.7
Net Amount	543.4	561.1
Total Derivatives
GROSS RECOGNIZED ASSETS	3,387.7	2,563.5
Derivative Assets	1,049.7	1,206.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	16.8	15.4
Net Amount	1,032.9	1,191.0
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	707.8	1,031.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	707.8	1,031.2
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	707.8	1,031.2
Other Assets (excluding Derivative Assets)	7,400.0	4,600.0
Federal funds sold	5.0	134.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	2,691.1	1,902.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2,334.1	1,308.8
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	357.0	593.5
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	16.5	12.7
Net Amount	340.5	580.8
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	151.9	71.6
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3.9	22.6
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	148.0	49.0
Net Amount	148.0	49.0
Interest Rate Swaps | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1.0	24.5
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	24.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1.0	0.1
Net Amount	$ 1.0	0.1
Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS		1.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET		1.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET		0.0
Net Amount		$ 0.0