Derivative Financial Instruments (Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value	$ 311,077.0	$ 301,494.9
Fair value asset	2,011.7	1,999.7
Total Gross Derivatives, asset	2,466.6	2,563.5
Less: Netting	1,114.9	1,357.1
Derivative Asset	1,351.7	1,206.4
Fair value liability	1,849.6	1,947.0
Derivative Liabilities	2,226.6	2,388.4
Less: Netting	1,507.8	1,796.3
Derivative Liabilities	718.8	592.1
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	9,107.1	11,313.7
Fair value asset	203.4	336.2
Fair value liability	40.9	96.8
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	4,470.0	4,590.4
Fair value asset	17.6	29.8
Fair value liability	20.5	23.3
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	301,969.9	290,181.2
Fair value asset	2,263.2	2,227.3
Fair value liability	2,185.7	2,291.6
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
Notional Value	766.5	605.6
Fair value asset	2.0	1.8
Fair value liability	39.5	33.0
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	180.3	122.2
Fair value asset	2.0	0.5
Fair value liability	1.8	0.2
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
Notional Value	586.2	483.4
Fair value asset	0.0	1.3
Fair value liability	37.7	32.8
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
Notional Value	301,203.4	289,575.6
Fair value asset	2,261.2	2,225.5
Fair value liability	2,146.2	2,258.6
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	292,204.9	281,864.4
Fair value asset	2,101.4	2,159.4
Fair value liability	2,060.5	2,190.0
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	8,998.5	7,711.2
Fair value asset	159.8	66.1
Fair value liability	85.7	68.6
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	1,528.4	2,648.2
Fair value asset	32.5	13.8
Fair value liability	13.7	57.8
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	450.0	600.0
Fair value asset	0.2	0.2
Fair value liability	0.2	1.2
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,658.7	3,475.1
Fair value asset	153.1	292.4
Fair value liability	$ 6.5	$ 14.5

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