Securities (Investment Security Gains and Losses by Security Type) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ 0.0	$ (189.4)
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	(34.8)
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	(23.0)
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	0.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	(4.2)
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	(48.2)
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	0.0	(56.6)
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ 0.0	$ (22.6)