Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
NOTIONAL VALUE	$ 301,494.9	$ 337,617.7
ASSET	1,999.7	2,203.3
Asset, Total Gross Derivative	2,563.5	2,654.1
Asset, Netting	1,357.1	1,860.0
Derivative Assets	1,206.4	794.1
LIABILITY	1,947.0	2,003.1
Derivative Liabilities	2,388.4	2,829.0
Liability, Netting	1,796.3	1,621.4
Derivative Liabilities	592.1	1,207.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	11,313.7	11,698.4
ASSET	336.2	60.9
LIABILITY	96.8	211.9
Derivatives Designated as Hedging under GAAP \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,590.4	4,473.1
ASSET	29.8	31.7
LIABILITY	23.3	18.2
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	2,648.2	3,289.0
ASSET	13.8	28.4
LIABILITY	57.8	13.0
Derivatives Designated as Hedging under GAAP \| Cash Flow Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	600.0	925.0
ASSET	0.2	0.2
LIABILITY	1.2	1.2
Derivatives Designated as Hedging under GAAP \| Net Investment Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,475.1	3,011.3
ASSET	292.4	0.6
LIABILITY	14.5	179.5
Total Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	290,181.2	325,919.3
ASSET	2,227.3	2,593.2
LIABILITY	2,291.6	2,617.1
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	605.6	618.8
ASSET	1.8	1.1
LIABILITY	33.0	30.5
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	122.2	214.1
ASSET	0.5	1.1
LIABILITY	0.2	0.1
Derivatives Not Designated as Hedging under GAAP \| Fair Value Hedges \| Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	483.4	404.7
ASSET	1.3	0.0
LIABILITY	32.8	30.4
Client-Related and Trading Derivatives \| Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	289,575.6	325,300.5
ASSET	2,225.5	2,592.1
LIABILITY	2,258.6	2,586.6
Client-Related and Trading Derivatives \| Fair Value Hedges \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	281,864.4	317,882.5
ASSET	2,159.4	2,527.0
LIABILITY	2,190.0	2,522.5
Client-Related and Trading Derivatives \| Fair Value Hedges \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	7,711.2	7,418.0
ASSET	66.1	65.1
LIABILITY	$ 68.6	$ 64.1

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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