Derivative Financial Instruments (Notional and Fair Value Amounts of Derivative Financial Instruments) (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Derivative [Line Items]
NOTIONAL VALUE	$ 413,689.4	$ 393,391.8
Derivative Assets	2,935.3	5,358.5
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,470.4	1,910.4
Derivative Assets	1,464.9	3,448.1
LIABILITY	3,183.5	5,158.4
Less: Liability, Netting	1,427.1	4,199.6
Derivative Liabilities	$ 1,756.4	$ 958.8
Derivative assets, location	Other Assets	Other Assets
Derivative liabilities, location	Other Liabilities	Other Liabilities
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	$ 14,390.9	$ 15,137.7
Derivative Assets	216.2	426.7
LIABILITY	336.9	183.6
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,706.3	9,706.3
Derivative Assets	212.2	189.4
LIABILITY	190.0	159.3
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	710.8	872.8
Derivative Assets	3.4	40.8
LIABILITY	17.0	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,973.8	4,558.6
Derivative Assets	0.6	196.5
LIABILITY	129.9	24.3
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	399,298.5	378,254.1
Derivative Assets	2,719.1	4,931.8
LIABILITY	2,846.6	4,974.8
Non-Designated Risk Management Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	569.5	513.7
Derivative Assets	0.0	0.0
LIABILITY	29.5	27.2
Non-Designated Risk Management Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.7	1.7
Derivative Assets	0.0	0.0
LIABILITY	0.0	0.0
Non-Designated Risk Management Derivatives | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	567.8	512.0
Derivative Assets	0.0	0.0
LIABILITY	29.5	27.2
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	398,729.0	377,740.4
Derivative Assets	2,719.1	4,931.8
LIABILITY	2,817.1	4,947.6
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	383,669.2	362,658.7
Derivative Assets	2,570.9	4,760.0
LIABILITY	2,603.0	4,685.5
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	15,059.8	15,081.7
Derivative Assets	148.2	171.8
LIABILITY	$ 214.1	$ 262.1