Securities (Investment Security Gains and Losses) (Details) - USD ($) $ in Millions	1 Months Ended	11 Months Ended	12 Months Ended
	Jan. 31, 2023	Nov. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Gross Realized Debt Securities Gains	$ 6.9		$ 10.5	$ 0.0	$ 1.4
Gross Realized Debt Securities Losses		$ 176.4	(180.0)	(214.0)	(1.7)
Investment Security Gains (Losses), net(1)			$ (169.5)	$ (214.0)	$ (0.3)