Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	$ 1,622.8	$ 4,162.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,108.5	1,910.4
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	514.3	2,252.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.9	23.6
NET AMOUNT	511.4	2,228.8
Total Derivatives Not Subject to a Master Netting Arrangement
Total Derivatives Not Subject to a Master Netting Arrangement	460.6	1,195.7
Total Derivatives
GROSS RECOGNIZED ASSETS	2,083.4	5,358.5
Derivative Assets	974.9	3,448.1
NET AMOUNT	972.0	3,424.5
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	66,752.9	65,338.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	64,896.6	64,912.8
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,856.3	426.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	1,856.3	426.0
NET AMOUNT	0.0	0.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1,288.9	3,801.6
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,020.2	1,745.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	268.7	2,056.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.9	23.6
NET AMOUNT	265.8	2,032.8
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	333.6	359.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	88.0	165.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	245.6	194.1
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNT	245.6	194.1
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS		1.9
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET		0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET		1.9
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET		0.0
NET AMOUNT		$ 1.9
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	0.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	0.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0
NET AMOUNT	$ 0.0