Securities (Investment Security Gains and Losses by Security Type) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ 0.0	$ 0.0	$ (189.3)	$ 6.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	(34.8)	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	0.0	9.8
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	(23.0)	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	0.0	(1.2)
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	(4.2)	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	(48.2)	0.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0	(56.5)	(0.8)
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ 0.0	$ 0.0	$ (22.6)	$ (0.9)