Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative [Line Items]
NOTIONAL VALUE	$ 335,008.9	$ 310,684.3
FAIR VALUE, ASSET	2,470.7	3,151.2
Total Gross Derivatives, asset	3,413.0	3,732.4
Less: Netting	1,553.1	2,810.7
Derivative Assets	1,859.9	921.7
FAIR VALUE, LIABILITY	3,023.9	2,548.9
Derivative Liabilities	3,316.4	3,654.1
Less: Netting	2,409.9	1,865.9
Derivative Liabilities	906.5	1,788.2
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	10,320.5	8,537.9
FAIR VALUE, ASSET	260.1	349.4
FAIR VALUE, LIABILITY	44.4	51.1
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	5,566.1	4,622.0
FAIR VALUE, ASSET	82.8	58.5
FAIR VALUE, LIABILITY	43.2	32.7
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	324,688.4	302,146.4
FAIR VALUE, ASSET	3,152.9	3,383.0
FAIR VALUE, LIABILITY	3,272.0	3,603.0
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	818.6	773.6
FAIR VALUE, ASSET	0.7	0.4
FAIR VALUE, LIABILITY	29.0	34.9
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	50.5	55.9
FAIR VALUE, ASSET	0.7	0.1
FAIR VALUE, LIABILITY	0.1	0.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	768.1	717.7
FAIR VALUE, ASSET	0.0	0.3
FAIR VALUE, LIABILITY	28.9	34.8
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	323,869.8	301,372.8
FAIR VALUE, ASSET	3,152.2	3,382.6
FAIR VALUE, LIABILITY	3,243.0	3,568.1
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	311,789.1	288,994.6
FAIR VALUE, ASSET	2,871.1	3,219.1
FAIR VALUE, LIABILITY	2,818.9	3,169.0
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	12,080.7	12,378.2
FAIR VALUE, ASSET	281.1	163.5
FAIR VALUE, LIABILITY	424.1	399.1
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	934.4	150.9
FAIR VALUE, ASSET	50.5	7.4
FAIR VALUE, LIABILITY	0.0	5.7
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,820.0	3,765.0
FAIR VALUE, ASSET	126.8	283.5
FAIR VALUE, LIABILITY	$ 1.2	$ 12.7