Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
NOTIONAL VALUE	$ 330,607.7	$ 346,752.1
Assets	2,004.0	4,097.2
Total Gross Derivatives, asset	2,816.3	4,558.2
Less: Netting	886.8	3,507.8
Derivative Assets	1,929.5	1,050.4
Liabilities	2,277.2	3,738.0
Derivative Liabilities	2,550.9	4,882.8
Less: Netting	2,088.3	2,817.1
Derivative Liabilities	462.6	2,065.7
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	10,441.2	14,802.3
Assets	155.5	23.8
Liabilities	35.2	321.9
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,507.2	4,717.6
Assets	8.9	8.2
Liabilities	7.2	10.2
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	320,166.5	331,949.8
Assets	2,660.8	4,534.4
Liabilities	2,515.7	4,560.9
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	891.8	813.1
Assets	0.7	0.1
Liabilities	38.6	35.4
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	95.0	67.7
Assets	0.7	0.1
Liabilities	0.2	0.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	796.8	745.4
Assets	0.0	0.0
Liabilities	38.4	35.3
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	319,274.7	331,136.7
Assets	2,660.1	4,534.3
Liabilities	2,477.1	4,525.5
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	308,510.8	320,563.4
Assets	2,461.6	4,245.1
Liabilities	2,378.1	4,410.7
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	10,763.9	10,573.3
Assets	198.5	289.2
Liabilities	99.0	114.8
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	2,131.5	6,554.4
Assets	80.8	15.4
Liabilities	16.1	104.0
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	50.0
Assets	0.0	0.1
Liabilities	0.0	0.0
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,802.5	3,480.3
Assets	65.8	0.1
Liabilities	$ 11.9	$ 207.7