Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
LOANS
Balance at Beginning of Period	$ 178.7	$ 144.3	$ 138.4
Charge-Offs	(15.5)	(7.5)	(6.0)
Recoveries	4.2	3.7	10.2
Net Recoveries (Charge-Offs)	(11.3)	(3.8)	4.2
Provision for Credit Losses	0.6	38.2	1.7
Balance at End of Period	168.0	178.7	144.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	26.9	38.5	34.1
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	3.5	(11.6)	4.4
Balance at End of Period	30.4	26.9	38.5
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	12.7	16.0	11.2
Charge-Offs	0.0	(1.2)	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	(1.2)	0.0
Provision for Credit Losses	(6.2)	(2.1)	4.8
Balance at End of Period	6.5	12.7	16.0
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.9	0.8	1.0
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for (Release of) Credit Losses	0.1	0.1	(0.2)
Balance at End of Period	1.0	0.9	0.8
TOTAL
Balance at Beginning of Period	219.2	199.6	184.7
Charge-Offs	(15.5)	(8.7)	(6.0)
Recoveries	4.2	3.7	10.2
Net Recoveries (Charge-Offs)	(11.3)	(5.0)	4.2
Provision for (Release of) Credit Losses	(2.0)	24.6	10.7
Balance at End of Period	205.9	219.2	199.6
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.9	1.9	1.4
Charge-Offs		0.0
Recoveries		0.0
Provision for Credit Losses	(0.6)	(1.0)	0.5
Balance at End of Period	$ 0.3	0.9	1.9
TOTAL
Provision for (Release of) Credit Losses		$ (0.1)	$ 1.3