Securities (Gains (Losses) on Debt Securities) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Gain (Loss) on Securities [Line Items]
Gross Realized Debt Securities Gains	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Gross Realized Debt Securities Losses	(73.9)	0.0	(73.9)	0.0
Investment Security Gains (Losses), net	(73.9)	0.0	(73.9)	0.0
U.S. Governments
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(4.5)	0.0	(4.5)	0.0
Obligations of States and Political Subdivisions
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(7.9)	0.0	(7.9)	0.0
Government Sponsored Agency
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(26.0)	0.0	(26.0)	0.0
Non-U.S. Government
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(4.1)	0.0	(4.1)	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(21.3)	0.0	(21.3)	0.0
Other Asset-Backed
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	(0.7)	0.0	(0.7)	0.0
Commercial Mortgage-Backed
Gain (Loss) on Securities [Line Items]
Investment Security Gains (Losses), net	$ (9.4)	$ 0.0	$ (9.4)	$ 0.0