Securities (Investment Security Gains and Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Gross Realized Debt Securities Gains	$ 0.0	$ 0.0	$ 185.2	$ 213.0
Gross Realized Debt Securities Losses	0.0	0.0	(374.5)	(206.1)
Investment Security Gains (Losses), net	$ 0.0	$ 0.0	$ (189.3)	$ 6.9