Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Derivative [Line Items]
NOTIONAL VALUE	$ 421,558.2	$ 393,391.8
Derivative Assets	2,094.3	5,358.5
Less: Asset, Netting	1,779.2	1,910.4
Derivative Asset	315.1	3,448.1
LIABILITY	2,409.6	5,158.4
Less: Liability, Netting	1,175.7	4,199.6
Derivative Liabilities	$ 1,233.9	$ 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	$ 16,943.9	$ 15,137.7
Derivative Assets	54.8	426.7
LIABILITY	351.8	183.6
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,897.4	9,706.3
Derivative Assets	16.5	189.4
LIABILITY	0.0	159.3
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1,312.0	872.8
Derivative Assets	29.6	40.8
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,734.5	4,558.6
Derivative Assets	8.7	196.5
LIABILITY	351.8	24.3
Total Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	404,614.3	378,254.1
Derivative Assets	2,039.5	4,931.8
LIABILITY	2,057.8	4,974.8
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	608.2	513.7
Derivative Assets	0.7	0.0
LIABILITY	31.3	27.2
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.6	1.7
Derivative Assets	0.0	0.0
LIABILITY	0.0	0.0
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	606.6	512.0
Derivative Assets	0.7	0.0
LIABILITY	31.3	27.2
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	404,006.1	377,740.4
Derivative Assets	2,038.8	4,931.8
LIABILITY	2,026.5	4,947.6
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	392,874.0	362,658.7
Derivative Assets	1,950.5	4,760.0
LIABILITY	1,896.1	4,685.5
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	11,132.1	15,081.7
Derivative Assets	88.3	171.8
LIABILITY	$ 130.4	$ 262.1