Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
NOTIONAL VALUE	$ 286,102.8	$ 310,600.7
Assets	2,767.6	2,844.0
Total Gross Derivatives, asset	3,329.6	3,387.7
Less: Netting	2,259.1	2,338.0
Derivative Assets	1,070.5	1,049.7
Liabilities	2,619.8	2,312.4
Derivative Liabilities	3,025.8	3,313.0
Less: Netting	2,370.7	1,618.4
Derivative Liabilities	655.1	1,694.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	12,613.2	9,273.5
Assets	99.4	102.7
Liabilities	30.9	44.5
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	5,513.2	4,538.2
Assets	9.6	20.3
Liabilities	15.6	20.9
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	273,489.6	301,327.2
Assets	3,230.2	3,285.0
Liabilities	2,994.9	3,268.5
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	874.1	816.8
Assets	1.8	0.9
Liabilities	32.8	34.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	215.8	176.5
Assets	1.8	0.9
Liabilities	1.5	0.7
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	658.3	640.3
Assets	0.0	0.0
Liabilities	31.3	33.4
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	272,615.5	300,510.4
Assets	3,228.4	3,284.1
Liabilities	2,962.1	3,234.4
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	262,270.0	291,533.6
Assets	2,881.2	3,151.7
Liabilities	2,823.1	3,158.1
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,345.5	8,976.8
Assets	347.2	132.4
Liabilities	139.0	76.3
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,002.5	1,661.5
Assets	29.8	8.5
Liabilities	10.8	11.5
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	150.0	200.0
Assets	0.5	0.2
Liabilities	0.0	0.2
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	2,947.5	2,873.8
Assets	59.5	73.7
Liabilities	$ 4.5	$ 11.9