Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
NOTIONAL VALUE	$ 310,600.7	$ 301,494.9
ASSET	2,844.0	1,999.7
Asset, Total Gross Derivative	3,387.7	2,563.5
Less: Asset, Netting	2,338.0	1,357.1
Derivative Assets	1,049.7	1,206.4
LIABILITY	2,312.4	1,947.0
Derivative Liabilities	3,313.0	2,388.4
Less: Liability, Netting	1,618.4	1,796.3
Derivative Liabilities	1,694.6	592.1
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	9,273.5	11,313.7
ASSET	102.7	336.2
LIABILITY	44.5	96.8
Derivatives Designated as Hedging under GAAP \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,538.2	4,590.4
ASSET	20.3	29.8
LIABILITY	20.9	23.3
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1,661.5	2,648.2
ASSET	8.5	13.8
LIABILITY	11.5	57.8
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	200.0	600.0
ASSET	0.2	0.2
LIABILITY	0.2	1.2
Derivatives Designated as Hedging under GAAP \| Net Investment Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	2,873.8	3,475.1
ASSET	73.7	292.4
LIABILITY	11.9	14.5
Total Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	301,327.2	290,181.2
ASSET	3,285.0	2,227.3
LIABILITY	3,268.5	2,291.6
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	816.8	605.6
ASSET	0.9	1.8
LIABILITY	34.1	33.0
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	176.5	122.2
ASSET	0.9	0.5
LIABILITY	0.7	0.2
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Interest Rate Swaps Exchange Cleared
Derivative [Line Items]
NOTIONAL VALUE	640.3	483.4
ASSET	0.0	1.3
LIABILITY	33.4	32.8
Client-Related and Trading Derivatives \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	300,510.4	289,575.6
ASSET	3,284.1	2,225.5
LIABILITY	3,234.4	2,258.6
Client-Related and Trading Derivatives \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	291,533.6	281,864.4
ASSET	3,151.7	2,159.4
LIABILITY	3,158.1	2,190.0
Client-Related and Trading Derivatives \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	8,976.8	7,711.2
ASSET	132.4	66.1
LIABILITY	$ 76.3	$ 68.6

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