Derivative Financial Instruments (Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 304,387.3	$ 301,494.9
Fair value asset	1,232.5	1,999.7
Total Gross Derivatives, asset	1,746.6	2,563.5
Less: Netting	979.9	1,357.1
Derivative Asset	766.7	1,206.4
Fair value liability	1,285.0	1,947.0
Derivative Liabilities	1,606.0	2,388.4
Less: Netting	1,123.7	1,796.3
Derivative Liabilities	482.3	592.1
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	9,848.6	11,313.7
Fair value asset	103.1	336.2
Fair value liability	36.8	96.8
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	4,479.8	4,590.4
Fair value asset	23.9	29.8
Fair value liability	19.9	23.3
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	294,538.7	290,181.2
Fair value asset	1,643.5	2,227.3
Fair value liability	1,569.2	2,291.6
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
Notional Value	677.1	605.6
Fair value asset	1.2	1.8
Fair value liability	36.4	33.0
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	117.3	122.2
Fair value asset	1.2	0.5
Fair value liability	0.5	0.2
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
Notional Value	559.8	483.4
Fair value asset	0.0	1.3
Fair value liability	35.9	32.8
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
Notional Value	293,861.6	289,575.6
Fair value asset	1,642.3	2,225.5
Fair value liability	1,532.8	2,258.6
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	286,240.1	281,864.4
Fair value asset	1,561.8	2,159.4
Fair value liability	1,474.0	2,190.0
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	7,621.5	7,711.2
Fair value asset	80.5	66.1
Fair value liability	58.8	68.6
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,259.1	2,648.2
Fair value asset	42.5	13.8
Fair value liability	10.5	57.8
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	600.0	600.0
Fair value asset	0.2	0.2
Fair value liability	0.9	1.2
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,509.7	3,475.1
Fair value asset	36.5	292.4
Fair value liability	$ 5.5	$ 14.5

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

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