Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (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
Commercial
LOANS
Balance at Beginning of Period	146.8	116.2	105.6
Charge-Offs	15.1	5.7	5.3
Recoveries	0.0	0.2	2.7
Net Recoveries (Charge-Offs)	(15.1)	(5.5)	(2.6)
Provision for Credit Losses	6.8	36.1	13.2
Balance at End of Period	138.5	146.8	116.2
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	24.9	36.3	31.4
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	3.4	(11.4)	4.9
Balance at End of Period	28.3	24.9	36.3
Personal
LOANS
Balance at Beginning of Period	31.9	28.1	32.8
Charge-Offs	0.4	1.8	0.7
Recoveries	4.2	3.5	7.5
Net Recoveries (Charge-Offs)	3.8	1.7	6.8
Provision for Credit Losses	(6.2)	2.1	(11.5)
Balance at End of Period	29.5	31.9	28.1
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.0	2.2	2.7
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	0.1	(0.2)	(0.5)
Balance at End of Period	$ 2.1	$ 2.0	$ 2.2