Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
LOANS
Balance at Beginning of Period	$ 168.0	$ 178.7	$ 178.7
Charge-Offs	(0.3)	(11.1)	(15.5)
Recoveries	0.5	0.7
Net Recoveries (Charge-Offs)	0.2	(10.4)
Provision for Credit Losses	(1.1)	(5.9)
Balance at End of Period	167.1	162.4	168.0
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	30.4	26.9	26.9
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	2.4	(1.7)
Balance at End of Period	32.8	25.2	30.4
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	6.5	12.7	12.7
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.2	(0.3)
Balance at End of Period	6.7	12.4	6.5
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1.0	0.9	0.9
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(0.3)	0.1
Balance at End of Period	0.7	1.0	1.0
TOTAL
Balance at Beginning of Period	205.9	219.2	219.2
Charge-Offs	(0.3)	(11.1)
Recoveries	0.5	0.7
Net Recoveries (Charge-Offs)	0.2	(10.4)
Provision for credit losses	1.2	(7.8)
Balance at End of Period	207.3	201.0	205.9
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.3	0.9	0.9
Provision for Credit Losses	0.0	(0.1)
Balance at End of Period	0.3	0.8	$ 0.3
TOTAL
Provision for credit losses	$ (0.2)	$ (0.7)