Derivative Financial Instruments (Notional and Fair Value Amounts of Derivative Financial Instruments) (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Derivative [Line Items]
NOTIONAL VALUE	$ 436,284.4	$ 393,391.8
Derivative Assets	2,083.4	5,358.5
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,108.5	1,910.4
Derivative Assets	974.9	3,448.1
LIABILITY	2,455.1	5,158.4
Less: Liability, Netting	1,444.5	4,199.6
Derivative Liabilities	$ 1,010.6	$ 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	$ 14,222.2	$ 15,137.7
Derivative Assets	241.5	426.7
LIABILITY	568.8	183.6
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,647.4	9,706.3
Derivative Assets	236.5	189.4
LIABILITY	219.2	159.3
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	872.8
Derivative Assets	0.0	40.8
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,574.8	4,558.6
Derivative Assets	5.0	196.5
LIABILITY	349.6	24.3
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	422,062.2	378,254.1
Derivative Assets	1,841.9	4,931.8
LIABILITY	1,886.3	4,974.8
Non-Designated Risk Management Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	598.1	513.7
Derivative Assets	0.3	0.0
LIABILITY	19.1	27.2
Non-Designated Risk Management Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.6	1.7
Derivative Assets	0.0	0.0
LIABILITY	0.1	0.0
Non-Designated Risk Management Derivatives | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	596.5	512.0
Derivative Assets	0.3	0.0
LIABILITY	19.0	27.2
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	421,464.1	377,740.4
Derivative Assets	1,841.6	4,931.8
LIABILITY	1,867.2	4,947.6
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	407,908.8	362,658.7
Derivative Assets	1,722.3	4,760.0
LIABILITY	1,702.7	4,685.5
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	13,555.3	15,081.7
Derivative Assets	119.3	171.8
LIABILITY	$ 164.5	$ 262.1