Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
NOTIONAL VALUE	$ 281,329.7	$ 310,600.7
Assets	2,098.0	2,844.0
Total Gross Derivatives, asset	2,613.2	3,387.7
Less: Netting	1,408.3	2,338.0
Derivative Assets	1,204.9	1,049.7
Liabilities	2,143.8	2,312.4
Derivative Liabilities	2,500.8	3,313.0
Less: Netting	1,603.3	1,618.4
Derivative Liabilities	897.5	1,694.6
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	14,555.8	9,273.5
Assets	45.3	102.7
Liabilities	131.1	44.5
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	5,357.8	4,538.2
Assets	10.2	20.3
Liabilities	18.9	20.9
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	266,773.9	301,327.2
Assets	2,567.9	3,285.0
Liabilities	2,369.7	3,268.5
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	760.7	816.8
Assets	2.0	0.9
Liabilities	32.9	34.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	79.2	176.5
Assets	2.0	0.9
Liabilities	0.6	0.7
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	681.5	640.3
Assets	0.0	0.0
Liabilities	32.3	33.4
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	266,013.2	300,510.4
Assets	2,565.9	3,284.1
Liabilities	2,336.8	3,234.4
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	255,799.4	291,533.6
Assets	2,242.7	3,151.7
Liabilities	2,213.5	3,158.1
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,213.8	8,976.8
Assets	323.2	132.4
Liabilities	123.3	76.3
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	5,882.7	1,661.5
Assets	14.0	8.5
Liabilities	28.9	11.5
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	150.0	200.0
Assets	0.3	0.2
Liabilities	0.0	0.2
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,165.3	2,873.8
Assets	20.8	73.7
Liabilities	$ 83.3	$ 11.9