Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 24,920.4	$ 25,036.1
GROSS UNREALIZED GAINS	2.9	1.4
GROSS UNREALIZED LOSSES	2,473.6	2,158.2
FAIR VALUE	22,449.7	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	2,577.7	2,565.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	177.9	149.8
FAIR VALUE	2,399.8	2,415.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,159.9	9,407.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1,405.6	1,076.0
FAIR VALUE	7,754.3	8,331.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,711.8	3,234.0
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	126.6	133.8
FAIR VALUE	3,585.3	3,100.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	639.1	713.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	38.0	45.4
FAIR VALUE	601.1	667.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,116.9	2,530.3
GROSS UNREALIZED GAINS	0.4	0.3
GROSS UNREALIZED LOSSES	142.0	158.7
FAIR VALUE	1,975.3	2,371.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	405.4	35.9
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	405.5	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,443.8	5,703.3
GROSS UNREALIZED GAINS	1.3	1.0
GROSS UNREALIZED LOSSES	400.3	436.1
FAIR VALUE	5,044.8	5,268.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	246.6	263.7
GROSS UNREALIZED GAINS	1.0	0.0
GROSS UNREALIZED LOSSES	0.2	1.0
FAIR VALUE	247.4	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	0.7
FAIR VALUE	36.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	581.6	532.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	182.3	157.4
FAIR VALUE	$ 399.3	$ 375.2