Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 23,497.9	$ 26,221.7
GROSS UNREALIZED GAINS	5.6	6.9
GROSS UNREALIZED LOSSES	1,862.1	1,755.6
FAIR VALUE	21,641.4	24,473.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,571.6	2,563.9
GROSS UNREALIZED GAINS	0.3	0.5
GROSS UNREALIZED LOSSES	89.5	72.4
FAIR VALUE	2,482.4	2,492.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,223.3	9,355.3
GROSS UNREALIZED GAINS	1.6	2.3
GROSS UNREALIZED LOSSES	1,099.2	1,012.4
FAIR VALUE	8,125.7	8,345.2
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,742.1	4,789.1
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	92.3	90.7
FAIR VALUE	2,649.9	4,698.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	610.3	646.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	24.7	28.2
FAIR VALUE	585.6	617.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,110.3	2,208.6
GROSS UNREALIZED GAINS	0.7	0.3
GROSS UNREALIZED LOSSES	103.8	108.3
FAIR VALUE	2,007.2	2,100.6
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	403.2	585.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.2	0.7
FAIR VALUE	403.0	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,024.5	5,245.5
GROSS UNREALIZED GAINS	1.6	3.2
GROSS UNREALIZED LOSSES	289.4	294.9
FAIR VALUE	4,736.7	4,953.8
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	193.7	214.2
GROSS UNREALIZED GAINS	1.3	0.4
GROSS UNREALIZED LOSSES	0.2	0.2
FAIR VALUE	194.8	214.4
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37.6	37.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.3	0.8
FAIR VALUE	37.3	36.8
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	581.3	576.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	162.5	147.0
FAIR VALUE	$ 418.8	$ 429.3