Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 24,716.2	$ 25,036.1
GROSS UNREALIZED GAINS	3.9	1.4
GROSS UNREALIZED LOSSES	1,897.0	2,158.2
FAIR VALUE	22,823.1	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	2,573.6	2,565.3
GROSS UNREALIZED GAINS	0.7	0.0
GROSS UNREALIZED LOSSES	74.9	149.8
FAIR VALUE	2,499.4	2,415.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,259.9	9,407.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	949.8	1,076.0
FAIR VALUE	8,310.1	8,331.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,772.2	3,234.0
GROSS UNREALIZED GAINS	0.7	0.1
GROSS UNREALIZED LOSSES	121.5	133.8
FAIR VALUE	2,651.4	3,100.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	673.0	713.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	42.6	45.4
FAIR VALUE	630.4	667.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,275.8	2,530.3
GROSS UNREALIZED GAINS	0.4	0.3
GROSS UNREALIZED LOSSES	148.9	158.7
FAIR VALUE	2,127.3	2,371.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	544.7	35.9
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	544.7	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,700.3	5,703.3
GROSS UNREALIZED GAINS	2.1	1.0
GROSS UNREALIZED LOSSES	397.1	436.1
FAIR VALUE	5,305.3	5,268.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	335.7	263.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.0	1.0
FAIR VALUE	334.7	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	543.4	532.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	160.4	157.4
FAIR VALUE	$ 383.0	$ 375.2