Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Derivative [Line Items]
NOTIONAL VALUE	$ 340,383.3	$ 310,684.3
ASSET	3,307.8	3,151.2
Asset, Total Gross Derivative	3,568.2	3,732.4
Less: Asset, Netting	3,126.7	2,810.7
Derivative Asset	441.5	921.7
LIABILITY	2,806.3	2,548.9
Derivative Liabilities	3,649.8	3,654.1
Less: Liability, Netting	2,205.4	1,865.9
Derivative Liabilities	1,444.4	1,788.2
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	12,573.7	8,537.9
ASSET	123.0	349.4
LIABILITY	145.1	51.1
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	7,042.7	4,622.0
ASSET	94.7	58.5
LIABILITY	71.0	32.7
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1,453.6	150.9
ASSET	15.0	7.4
LIABILITY	42.2	5.7
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,077.4	3,765.0
ASSET	13.3	283.5
LIABILITY	31.9	12.7
Total Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	327,809.6	302,146.4
ASSET	3,445.2	3,383.0
LIABILITY	3,504.7	3,603.0
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	888.6	773.6
ASSET	0.0	0.4
LIABILITY	25.5	34.9
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	20.7	55.9
ASSET	0.0	0.1
LIABILITY	0.1	0.1
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	867.9	717.7
ASSET	0.0	0.3
LIABILITY	25.4	34.8
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	326,921.0	301,372.8
ASSET	3,445.2	3,382.6
LIABILITY	3,479.2	3,568.1
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	313,336.9	288,994.6
ASSET	3,238.4	3,219.1
LIABILITY	3,181.0	3,169.0
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	13,584.1	12,378.2
ASSET	206.8	163.5
LIABILITY	$ 298.2	$ 399.1