Derivative Financial Instruments (Notional and Fair Value Amounts of Derivative Financial Instruments) (Detail) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Derivative [Line Items]
NOTIONAL VALUE	$ 534,543.7	$ 421,558.2
Derivative Assets	3,375.1	2,094.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,830.4	1,779.2
Derivative Assets	1,544.7	315.1
LIABILITY	3,321.3	2,409.6
Less: Liability, Netting	2,752.4	1,175.7
Derivative Liabilities	$ 568.9	$ 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	$ 35,755.3	$ 16,943.9
Derivative Assets	128.2	54.8
LIABILITY	11.4	351.8
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,316.6	10,897.4
Derivative Assets	0.0	16.5
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	1.6	0.0
LIABILITY	0.1	0.0
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	399.9	1,312.0
Derivative Assets	16.8	29.6
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	5,038.8	4,734.5
Derivative Assets	109.8	8.7
LIABILITY	11.3	351.8
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	498,788.4	404,614.3
Derivative Assets	3,246.9	2,039.5
LIABILITY	3,309.9	2,057.8
Non-Designated Risk Management Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	368.2	608.2
Derivative Assets	0.3	0.7
LIABILITY	23.1	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	0.0	0.0
Non-Designated Risk Management Derivatives | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	366.7	606.6
Derivative Assets	0.3	0.7
LIABILITY	23.1	31.3
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	498,420.2	404,006.1
Derivative Assets	3,246.6	2,038.8
LIABILITY	3,286.8	2,026.5
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,253.1	11,132.1
Derivative Assets	78.6	88.3
LIABILITY	138.7	130.4
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	489,167.1	392,874.0
Derivative Assets	3,168.0	1,950.5
LIABILITY	$ 3,148.1	$ 1,896.1