Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Derivative [Line Items]
NOTIONAL VALUE	$ 347,860.4	$ 340,241.0
Assets	2,545.8	1,902.7
Total Gross Derivatives, asset	3,266.9	2,347.4
Less: Netting	2,176.6	1,533.8
Derivative Assets	1,090.3	813.6
Liabilities	2,821.9	1,566.5
Derivative Liabilities	3,355.0	2,135.2
Less: Netting	2,430.4	1,283.5
Derivative Liabilities	924.6	851.7
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	8,387.2	12,290.2
Assets	90.7	252.7
Liabilities	37.8	33.6
Fair Value Hedges \| Derivatives Designated as Hedging under GAAP \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,336.2	4,430.5
Assets	18.5	7.6
Liabilities	13.9	8.7
Fair Value Hedges \| Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	339,473.2	327,950.8
Assets	3,176.2	2,094.7
Liabilities	3,317.2	2,101.6
Fair Value Hedges \| Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	835.1	848.4
Assets	0.8	0.2
Liabilities	35.4	38.1
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	81.5	109.9
Assets	0.8	0.2
Liabilities	0.1	0.6
Fair Value Hedges \| Non-Designated Risk Management Derivatives \| Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	753.6	738.5
Assets	0.0	0.0
Liabilities	35.3	37.5
Fair Value Hedges \| Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	338,638.1	327,102.4
Assets	3,175.4	2,094.5
Liabilities	3,281.8	2,063.5
Fair Value Hedges \| Client-Related and Trading Derivatives \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	327,233.7	315,532.3
Assets	3,112.2	1,962.1
Liabilities	3,095.1	1,973.3
Fair Value Hedges \| Client-Related and Trading Derivatives \| Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	11,404.4	11,570.1
Assets	63.2	132.4
Liabilities	186.7	90.2
Cash Flow Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	467.5	3,697.9
Assets	6.9	61.8
Liabilities	12.0	12.6
Net Investment Hedges \| Derivatives Designated as Hedging under GAAP \| Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,583.5	4,161.8
Assets	65.3	183.3
Liabilities	$ 11.9	$ 12.3

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