Securities (Investment Security Gains and Losses by Security Type) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ (169.5)	$ (214.0)	$ (0.3)
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	(29.9)	0.0	0.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	9.8	(95.8)	0.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	(73.2)	0.0	0.0
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	(7.6)	(67.3)	(0.3)
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	(58.6)	(17.1)	0.0
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	(0.9)	(33.8)	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Investment Security Gains (Losses), net	$ (9.1)	$ 0.0	$ 0.0