Derivative Financial Instruments (Notional and Fair Value Amounts of Derivative Financial Instruments) (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Derivative [Line Items]
NOTIONAL VALUE	$ 524,219.6	$ 421,558.2
Derivative Assets	3,857.7	2,094.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2,179.8	1,779.2
Derivative Assets	1,677.9	315.1
LIABILITY	3,793.5	2,409.6
Less: Liability, Netting	2,757.4	1,175.7
Derivative Liabilities	$ 1,036.1	$ 1,233.9
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	$ 36,839.6	$ 16,943.9
Derivative Assets	144.3	54.8
LIABILITY	6.1	351.8
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,652.4	10,897.4
Derivative Assets	17.5	16.5
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1,413.9	1,312.0
Derivative Assets	47.6	29.6
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	20,000.0	0.0
Derivative Assets	5.7	0.0
LIABILITY	0.3	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,773.3	4,734.5
Derivative Assets	73.5	8.7
LIABILITY	5.8	351.8
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	487,380.0	404,614.3
Derivative Assets	3,713.4	2,039.5
LIABILITY	3,787.4	2,057.8
Non-Designated Risk Management Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	491.5	608.2
Derivative Assets	0.0	0.7
LIABILITY	23.8	31.3
Non-Designated Risk Management Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.5	1.6
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	490.0	606.6
Derivative Assets	0.0	0.7
LIABILITY	23.7	31.3
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	486,888.5	404,006.1
Derivative Assets	3,713.4	2,038.8
LIABILITY	3,763.6	2,026.5
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	476,900.4	392,874.0
Derivative Assets	3,632.6	1,950.5
LIABILITY	3,633.5	1,896.1
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,988.1	11,132.1
Derivative Assets	80.8	88.3
LIABILITY	$ 130.1	$ 130.4