Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
NOTIONAL VALUE	$ 362,936.2	$ 340,383.3
FAIR VALUE, ASSET	1,498.4	3,307.8
Total Gross Derivatives, asset	1,833.8	3,568.2
Less: Netting	1,119.0	3,126.7
Derivative Assets	714.8	441.5
FAIR VALUE, LIABILITY	1,618.1	2,806.3
Derivative Liabilities	1,881.8	3,649.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	881.5	2,205.4
Derivative Liabilities	1,000.3	1,444.4
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	13,985.0	12,573.7
FAIR VALUE, ASSET	244.1	123.0
FAIR VALUE, LIABILITY	163.4	145.1
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,706.9	7,042.7
FAIR VALUE, ASSET	199.0	94.7
FAIR VALUE, LIABILITY	156.8	71.0
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	348,951.2	327,809.6
FAIR VALUE, ASSET	1,589.7	3,445.2
FAIR VALUE, LIABILITY	1,718.4	3,504.7
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	583.6	888.6
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	30.6	25.5
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1.7	20.7
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	0.0	0.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	581.9	867.9
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	30.6	25.4
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	348,367.6	326,921.0
FAIR VALUE, ASSET	1,589.7	3,445.2
FAIR VALUE, LIABILITY	1,687.8	3,479.2
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	332,750.4	313,336.9
FAIR VALUE, ASSET	1,379.2	3,238.4
FAIR VALUE, LIABILITY	1,371.9	3,181.0
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	15,617.2	13,584.1
FAIR VALUE, ASSET	210.5	206.8
FAIR VALUE, LIABILITY	315.9	298.2
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	1,453.6
FAIR VALUE, ASSET	0.0	15.0
FAIR VALUE, LIABILITY	0.0	42.2
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,278.1	4,077.4
FAIR VALUE, ASSET	45.1	13.3
FAIR VALUE, LIABILITY	$ 6.6	$ 31.9