Securities (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) investment	Dec. 31, 2023 USD ($) investment	Dec. 31, 2022 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Allowance for credit losses, available-for-sale securities	$ 0.2	$ 1.2
Allowance released	$ 1.0	$ 0.1
Number of securities in an unrealized loss position | investment	767	898
Debt securities, available-for-sale, unrealized loss position	$ 17,832.2	$ 19,435.3
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 255.1	$ 568.3
Percentage of debt securities, held-to-maturity, portfolio	1	1
Gross proceeds from sale of securities	$ 2,000.0	$ 5,200.0	$ 138.7
Moodys And Standard Poors A Or Higher Rating
Debt and Equity Securities, FV-NI [Line Items]
Percentage of debt securities, held-to-maturity, portfolio	0.96	0.97