Derivative Financial Instruments (Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value	$ 313,571.2	$ 310,600.7
Fair value asset	4,884.1	2,844.0
Total Gross Derivatives, asset	6,276.9	3,387.7
Less: Netting	2,065.8	2,338.0
Derivative Asset	4,211.1	1,049.7
Fair value liability	5,187.4	2,312.4
Derivative Liabilities	5,950.0	3,313.0
Less: Netting	4,890.2	1,618.4
Derivative Liabilities	1,059.8	1,694.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	8,994.7	9,273.5
Fair value asset	159.5	102.7
Fair value liability	33.7	44.5
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	4,527.2	4,538.2
Fair value asset	12.5	20.3
Fair value liability	18.3	20.9
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	304,576.5	301,327.2
Fair value asset	6,117.4	3,285.0
Fair value liability	5,916.3	3,268.5
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
Notional Value	745.6	816.8
Fair value asset	2.7	0.9
Fair value liability	28.4	34.1
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	195.0	176.5
Fair value asset	2.7	0.9
Fair value liability	1.9	0.7
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
Notional Value	550.6	640.3
Fair value asset	0.0	0.0
Fair value liability	26.5	33.4
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
Notional Value	303,830.9	300,510.4
Fair value asset	6,114.7	3,284.1
Fair value liability	5,887.9	3,234.4
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	293,879.0	291,533.6
Fair value asset	5,786.1	3,151.7
Fair value liability	5,746.1	3,158.1
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	9,951.9	8,976.8
Fair value asset	328.6	132.4
Fair value liability	141.8	76.3
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	1,544.2	1,661.5
Fair value asset	35.8	8.5
Fair value liability	14.9	11.5
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	150.0	200.0
Fair value asset	0.4	0.2
Fair value liability	0.0	0.2
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,773.3	2,873.8
Fair value asset	110.8	73.7
Fair value liability	$ 0.5	$ 11.9

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