Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
NOTIONAL VALUE	$ 397,798.9	$ 340,383.3
FAIR VALUE, ASSET	3,280.7	3,307.8
Total Gross Derivatives, asset	3,519.0	3,568.2
Less: Netting	2,773.1	3,126.7
Derivative Assets	745.9	441.5
FAIR VALUE, LIABILITY	2,758.3	2,806.3
Derivative Liabilities	3,615.8	3,649.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,819.9	2,205.4
Derivative Liabilities	1,795.9	1,444.4
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	14,811.6	12,573.7
FAIR VALUE, ASSET	317.1	123.0
FAIR VALUE, LIABILITY	394.1	145.1
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,706.3	7,042.7
FAIR VALUE, ASSET	308.9	94.7
FAIR VALUE, LIABILITY	289.7	71.0
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	382,987.3	327,809.6
FAIR VALUE, ASSET	3,201.9	3,445.2
FAIR VALUE, LIABILITY	3,221.7	3,504.7
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	460.0	888.6
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	27.8	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	458.3	867.9
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	27.8	25.4
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	382,527.3	326,921.0
FAIR VALUE, ASSET	3,201.9	3,445.2
FAIR VALUE, LIABILITY	3,193.9	3,479.2
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	367,149.3	313,336.9
FAIR VALUE, ASSET	3,044.7	3,238.4
FAIR VALUE, LIABILITY	2,979.5	3,181.0
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	15,378.0	13,584.1
FAIR VALUE, ASSET	157.2	206.8
FAIR VALUE, LIABILITY	214.4	298.2
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	454.6	1,453.6
FAIR VALUE, ASSET	1.5	15.0
FAIR VALUE, LIABILITY	0.5	42.2
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,650.7	4,077.4
FAIR VALUE, ASSET	6.7	13.3
FAIR VALUE, LIABILITY	$ 103.9	$ 31.9