Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
NOTIONAL VALUE	$ 346,752.1	$ 310,600.7
ASSET	4,097.2	2,844.0
Asset, Total Gross Derivative	4,558.2	3,387.7
Less: Asset, Netting	3,507.8	2,338.0
Derivative Assets	1,050.4	1,049.7
LIABILITY	3,738.0	2,312.4
Derivative Liabilities	4,882.8	3,313.0
Less: Liability, Netting	2,817.1	1,618.4
Derivative Liabilities	2,065.7	1,694.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	14,802.3	9,273.5
ASSET	23.8	102.7
LIABILITY	321.9	44.5
Derivatives Designated as Hedging under GAAP \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,717.6	4,538.2
ASSET	8.2	20.3
LIABILITY	10.2	20.9
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	6,554.4	1,661.5
ASSET	15.4	8.5
LIABILITY	104.0	11.5
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	50.0	200.0
ASSET	0.1	0.2
LIABILITY	0.0	0.2
Derivatives Designated as Hedging under GAAP \| Net Investment Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,480.3	2,873.8
ASSET	0.1	73.7
LIABILITY	207.7	11.9
Total Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	331,949.8	301,327.2
ASSET	4,534.4	3,285.0
LIABILITY	4,560.9	3,268.5
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	813.1	816.8
ASSET	0.1	0.9
LIABILITY	35.4	34.1
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	67.7	176.5
ASSET	0.1	0.9
LIABILITY	0.1	0.7
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	745.4	640.3
ASSET	0.0	0.0
LIABILITY	35.3	33.4
Client-Related and Trading Derivatives \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	331,136.7	300,510.4
ASSET	4,534.3	3,284.1
LIABILITY	4,525.5	3,234.4
Client-Related and Trading Derivatives \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	320,563.4	291,533.6
ASSET	4,245.1	3,151.7
LIABILITY	4,410.7	3,158.1
Client-Related and Trading Derivatives \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,573.3	8,976.8
ASSET	289.2	132.4
LIABILITY	$ 114.8	$ 76.3

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