Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 26,221.7	$ 25,036.1
GROSS UNREALIZED GAINS	6.9	1.4
GROSS UNREALIZED LOSSES	1,755.6	2,158.2
FAIR VALUE	24,473.0	22,879.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0.0	50.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	0.0	50.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,563.9	2,565.3
GROSS UNREALIZED GAINS	0.5	0.0
GROSS UNREALIZED LOSSES	72.4	149.8
FAIR VALUE	2,492.0	2,415.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,355.3	9,407.7
GROSS UNREALIZED GAINS	2.3	0.0
GROSS UNREALIZED LOSSES	1,012.4	1,076.0
FAIR VALUE	8,345.2	8,331.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,789.1	3,234.0
GROSS UNREALIZED GAINS	0.2	0.1
GROSS UNREALIZED LOSSES	90.7	133.8
FAIR VALUE	4,698.6	3,100.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	646.1	713.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	28.2	45.4
FAIR VALUE	617.9	667.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,208.6	2,530.3
GROSS UNREALIZED GAINS	0.3	0.3
GROSS UNREALIZED LOSSES	108.3	158.7
FAIR VALUE	2,100.6	2,371.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	585.1	35.9
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.7	0.0
FAIR VALUE	584.4	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,245.5	5,703.3
GROSS UNREALIZED GAINS	3.2	1.0
GROSS UNREALIZED LOSSES	294.9	436.1
FAIR VALUE	4,953.8	5,268.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	214.2	263.7
GROSS UNREALIZED GAINS	0.4	0.0
GROSS UNREALIZED LOSSES	0.2	1.0
FAIR VALUE	214.4	262.7
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37.6
GROSS UNREALIZED GAINS	0.0
GROSS UNREALIZED LOSSES	0.8
FAIR VALUE	36.8
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	576.3	532.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	147.0	157.4
FAIR VALUE	$ 429.3	$ 375.2