Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Derivative [Line Items]
NOTIONAL VALUE	$ 393,391.8	$ 340,383.3
Derivative Assets	5,358.5	3,568.2
Less: Asset, Netting	1,910.4	3,126.7
Derivative Asset	3,448.1	441.5
LIABILITY	5,158.4	3,649.8
Less: Liability, Netting	4,199.6	2,205.4
Derivative Liabilities	$ 958.8	$ 1,444.4
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	$ 15,137.7	$ 12,573.7
Derivative Assets	426.7	123.0
LIABILITY	183.6	145.1
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,706.3	7,042.7
Derivative Assets	189.4	94.7
LIABILITY	159.3	71.0
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	872.8	1,453.6
Derivative Assets	40.8	15.0
LIABILITY	0.0	42.2
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,558.6	4,077.4
Derivative Assets	196.5	13.3
LIABILITY	24.3	31.9
Total Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	378,254.1	327,809.6
Derivative Assets	4,931.8	3,445.2
LIABILITY	4,974.8	3,504.7
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	513.7	888.6
Derivative Assets	0.0	0.0
LIABILITY	27.2	25.5
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.7	20.7
Derivative Assets	0.0	0.0
LIABILITY	0.0	0.1
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	512.0	867.9
Derivative Assets	0.0	0.0
LIABILITY	27.2	25.4
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	377,740.4	326,921.0
Derivative Assets	4,931.8	3,445.2
LIABILITY	4,947.6	3,479.2
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	362,658.7	313,336.9
Derivative Assets	4,760.0	3,238.4
LIABILITY	4,685.5	3,181.0
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	15,081.7	13,584.1
Derivative Assets	171.8	206.8
LIABILITY	$ 262.1	$ 298.2