Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
NOTIONAL VALUE	$ 335,746.7	$ 346,752.1
Assets	2,509.4	4,097.2
Total Gross Derivatives, asset	3,206.6	4,558.2
Less: Netting	1,977.1	3,507.8
Derivative Assets	1,229.5	1,050.4
Liabilities	2,251.7	3,738.0
Derivative Liabilities	2,701.3	4,882.8
Less: Netting	2,090.3	2,817.1
Derivative Liabilities	611.0	2,065.7
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	14,460.7	14,802.3
Assets	380.2	23.8
Liabilities	42.9	321.9
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,513.7	4,717.6
Assets	10.0	8.2
Liabilities	13.7	10.2
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	321,286.0	331,949.8
Assets	2,826.4	4,534.4
Liabilities	2,658.4	4,560.9
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	852.5	813.1
Assets	0.7	0.1
Liabilities	34.7	35.4
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	131.0	67.7
Assets	0.7	0.1
Liabilities	0.1	0.1
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	721.5	745.4
Assets	0.0	0.0
Liabilities	34.6	35.3
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	320,433.5	331,136.7
Assets	2,825.7	4,534.3
Liabilities	2,623.7	4,525.5
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	309,731.3	320,563.4
Assets	2,630.1	4,245.1
Liabilities	2,513.7	4,410.7
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,702.2	10,573.3
Assets	195.6	289.2
Liabilities	110.0	114.8
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	6,464.9	6,554.4
Assets	296.7	15.4
Liabilities	20.4	104.0
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	50.0
Assets	0.0	0.1
Liabilities	0.0	0.0
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,482.1	3,480.3
Assets	73.5	0.1
Liabilities	$ 8.8	$ 207.7

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