Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Derivative [Line Items]
NOTIONAL VALUE	$ 347,592.9	$ 340,383.3
FAIR VALUE, ASSET	1,327.3	3,307.8
Total Gross Derivatives, asset	1,761.1	3,568.2
Less: Netting	914.2	3,126.7
Derivative Assets	846.9	441.5
FAIR VALUE, LIABILITY	1,630.2	2,806.3
Derivative Liabilities	1,817.0	3,649.8
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,070.1	2,205.4
Derivative Liabilities	746.9	1,444.4
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	13,718.0	12,573.7
FAIR VALUE, ASSET	207.6	123.0
FAIR VALUE, LIABILITY	420.9	145.1
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	9,466.4	7,042.7
FAIR VALUE, ASSET	182.4	94.7
FAIR VALUE, LIABILITY	420.1	71.0
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	333,874.9	327,809.6
FAIR VALUE, ASSET	1,553.5	3,445.2
FAIR VALUE, LIABILITY	1,396.1	3,504.7
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	930.4	888.6
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	21.0	25.5
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	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	930.4	867.9
FAIR VALUE, ASSET	0.0	0.0
FAIR VALUE, LIABILITY	21.0	25.4
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	332,944.5	326,921.0
FAIR VALUE, ASSET	1,553.5	3,445.2
FAIR VALUE, LIABILITY	1,375.1	3,479.2
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	317,051.6	313,336.9
FAIR VALUE, ASSET	1,331.9	3,238.4
FAIR VALUE, LIABILITY	1,317.3	3,181.0
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	15,892.9	13,584.1
FAIR VALUE, ASSET	221.6	206.8
FAIR VALUE, LIABILITY	57.8	298.2
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	164.9	1,453.6
FAIR VALUE, ASSET	4.0	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	4,086.7	4,077.4
FAIR VALUE, ASSET	21.2	13.3
FAIR VALUE, LIABILITY	$ 0.8	$ 31.9