Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 26,006.3	$ 25,036.1
GROSS UNREALIZED GAINS	1.8	1.4
GROSS UNREALIZED LOSSES	2,248.9	2,158.2
FAIR VALUE	23,759.2	22,879.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		50.0
GROSS UNREALIZED GAINS		0.0
GROSS UNREALIZED LOSSES		0.0
FAIR VALUE		50.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,573.0	2,565.3
GROSS UNREALIZED GAINS	0.2	0.0
GROSS UNREALIZED LOSSES	108.5	149.8
FAIR VALUE	2,464.7	2,415.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,354.2	9,407.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1,137.2	1,076.0
FAIR VALUE	8,217.0	8,331.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,987.3	3,234.0
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	145.5	133.8
FAIR VALUE	3,841.9	3,100.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	669.9	713.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	46.7	45.4
FAIR VALUE	623.2	667.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,278.7	2,530.3
GROSS UNREALIZED GAINS	0.3	0.3
GROSS UNREALIZED LOSSES	151.3	158.7
FAIR VALUE	2,127.7	2,371.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	588.2	35.9
GROSS UNREALIZED GAINS	0.2	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	588.4	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,645.8	5,703.3
GROSS UNREALIZED GAINS	0.8	1.0
GROSS UNREALIZED LOSSES	491.8	436.1
FAIR VALUE	5,154.8	5,268.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	288.0	263.7
GROSS UNREALIZED GAINS	0.2	0.0
GROSS UNREALIZED LOSSES	0.5	1.0
FAIR VALUE	287.7	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	583.6	532.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	166.6	157.4
FAIR VALUE	$ 417.0	$ 375.2