Derivative Financial Instruments (Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 308,667.6	$ 301,494.9
Fair value asset	1,512.1	1,999.7
Total Gross Derivatives, asset	1,891.8	2,563.5
Less: Netting	1,171.6	1,357.1
Derivative Asset	720.2	1,206.4
Fair value liability	1,416.4	1,947.0
Derivative Liabilities	1,815.5	2,388.4
Less: Netting	1,248.6	1,796.3
Derivative Liabilities	566.9	592.1
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	8,641.0	11,313.7
Fair value asset	84.4	336.2
Fair value liability	42.6	96.8
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	4,475.3	4,590.4
Fair value asset	22.4	29.8
Fair value liability	24.3	23.3
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	300,026.6	290,181.2
Fair value asset	1,807.4	2,227.3
Fair value liability	1,772.9	2,291.6
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
Notional Value	739.4	605.6
Fair value asset	0.5	1.8
Fair value liability	35.4	33.0
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	119.4	122.2
Fair value asset	0.5	0.5
Fair value liability	0.1	0.2
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
Notional Value	620.0	483.4
Fair value asset	0.0	1.3
Fair value liability	35.3	32.8
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
Notional Value	299,287.2	289,575.6
Fair value asset	1,806.9	2,225.5
Fair value liability	1,737.5	2,258.6
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	291,041.7	281,864.4
Fair value asset	1,679.1	2,159.4
Fair value liability	1,664.6	2,190.0
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	8,245.5	7,711.2
Fair value asset	127.8	66.1
Fair value liability	72.9	68.6
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	974.4	2,648.2
Fair value asset	13.7	13.8
Fair value liability	8.9	57.8
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	550.0	600.0
Fair value asset	0.2	0.2
Fair value liability	0.3	1.2
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,641.3	3,475.1
Fair value asset	48.1	292.4
Fair value liability	$ 9.1	$ 14.5

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

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