Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
NOTIONAL VALUE	$ 325,230.9	$ 346,752.1
Assets	2,152.5	4,097.2
Total Gross Derivatives, asset	3,004.2	4,558.2
Less: Netting	939.5	3,507.8
Derivative Assets	2,064.7	1,050.4
Liabilities	2,509.1	3,738.0
Derivative Liabilities	2,756.7	4,882.8
Less: Netting	2,333.9	2,817.1
Derivative Liabilities	422.8	2,065.7
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	10,114.7	14,802.3
Assets	176.3	23.8
Liabilities	28.3	321.9
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,503.2	4,717.6
Assets	17.6	8.2
Liabilities	9.3	10.2
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	315,116.2	331,949.8
Assets	2,827.9	4,534.4
Liabilities	2,728.4	4,560.9
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	863.6	813.1
Assets	1.1	0.1
Liabilities	36.7	35.4
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	104.5	67.7
Assets	1.1	0.1
Liabilities	0.0	0.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	759.1	745.4
Assets	0.0	0.0
Liabilities	36.7	35.3
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	314,252.6	331,136.7
Assets	2,826.8	4,534.3
Liabilities	2,691.7	4,525.5
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	302,703.2	320,563.4
Assets	2,649.6	4,245.1
Liabilities	2,593.7	4,410.7
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	11,549.4	10,573.3
Assets	177.2	289.2
Liabilities	98.0	114.8
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	1,762.5	6,554.4
Assets	12.5	15.4
Liabilities	13.5	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	3,849.0	3,480.3
Assets	146.2	0.1
Liabilities	$ 5.5	$ 207.7