Derivative Financial Instruments (Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value	$ 316,580.8	$ 337,617.7
Fair value asset	2,090.8	2,203.3
Total Gross Derivatives, asset	2,609.5	2,654.1
Less: Netting	1,622.8	1,860.0
Derivative Asset	986.7	794.1
Fair value liability	2,043.8	2,003.1
Total Gross Derivatives, liabilities	2,347.9	2,829.0
Less: Netting	1,631.5	1,621.4
Derivative Liabilities	716.4	1,207.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	12,233.1	11,698.4
Fair value asset	344.0	60.9
Fair value liability	44.4	211.9
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	4,643.7	4,473.1
Fair value asset	33.2	31.7
Fair value liability	26.3	18.2
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
Notional Value	304,347.7	325,919.3
Fair value asset	2,265.5	2,593.2
Fair value liability	2,303.5	2,617.1
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
Notional Value	675.1	618.8
Fair value asset	0.6	1.1
Fair value liability	39.1	30.5
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	129.5	214.1
Fair value asset	0.6	1.1
Fair value liability	2.0	0.1
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
Notional Value	545.6	404.7
Fair value asset	0.0	0.0
Fair value liability	37.1	30.4
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
Notional Value	303,672.6	325,300.5
Fair value asset	2,264.9	2,592.1
Fair value liability	2,264.4	2,586.6
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	296,062.5	317,882.5
Fair value asset	2,191.8	2,527.0
Fair value liability	2,161.9	2,522.5
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	7,610.1	7,418.0
Fair value asset	73.1	65.1
Fair value liability	102.5	64.1
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	3,806.6	3,289.0
Fair value asset	79.3	28.4
Fair value liability	12.6	13.0
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	725.0	925.0
Fair value asset	0.2	0.2
Fair value liability	2.1	1.2
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	3,057.8	3,011.3
Fair value asset	231.3	0.6
Fair value liability	$ 3.4	$ 179.5

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

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