Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (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
COMMERCIAL
LOANS
Balance at Beginning of Period	138.5	146.8	146.8
Charge-Offs	(0.3)	(11.0)	(15.1)
Recoveries	0.1	0.0
Net Recoveries (Charge-Offs)	(0.2)	(11.0)
Provision for Credit Losses	(4.7)	(4.1)
Balance at End of Period	133.6	131.7	138.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	28.3	24.9	24.9
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	(2.4)
Balance at End of Period	30.7	22.5	28.3
PERSONAL
LOANS
Balance at Beginning of Period	29.5	31.9	31.9
Charge-Offs	0.0	(0.1)	(0.4)
Recoveries	0.4	0.7
Net Recoveries (Charge-Offs)	0.4	0.6
Provision for Credit Losses	3.6	(1.8)
Balance at End of Period	33.5	30.7	29.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.1	2.0	2.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.0	0.7
Balance at End of Period	$ 2.1	$ 2.7	$ 2.1